John Quincy Adams took the oath of office on a book of Constitutional Law. How big was this book?

All we know about the book is it was a law book, and we only know this because John Quincy Adams mentioned it in his diary. We actually don't know if this was common practice at the time or not, because none of the other early presidents were as comprehensive diarists as Adams was, nor did they mention what book they took the oath on in any of their other writings. No journalist or other observer made any note of the book used at the other early inaugurations, either.

What John Quincy Adams wrote is pretty vague:

"...I repaired to the hall of the House of Representatives, and, after delivering from the Speaker's chair my inaugural address to a crowded auditory, I pronounced from a volume of the laws held up to me by John Marshall, Chief Justice of the United States, the oath faithfully to execute the office of President of the United States..."

We don't even know if it was a book of "constitutional law". Just a "volume of the laws". The context makes it sound like John Marshall brought it with him. If not, then it was likely a volume Marshall or someone else borrowed from the Library of Congress to use for the purpose. The Library of Congress had been established in 1800, but its law library collection of 174 titles was destroyed in 1814 during the War of 1812. Thomas Jefferson then arranged the purchase of his personal collection of 475 law titles: 318 were books published in England, while the rest were largely books on Virginia law. It's possible it was any of these books, or the other books the Library had collected since. The Library of Congress had also received copies of all federal laws and Supreme Court decisions by the time of the 1825 inauguration.

One possibility is that the book was a volume of the series now referred to as the United States Reports. Starting in 1790, there was an unofficial government position created called the Reporter of Decisions of the Supreme Court of the United States whose job it was to record all the Supreme Court decisions each session. This became an official position created by Congress in 1817. The Reporter was salaried, and they also published all these recorded Supreme Court decisions each session into a bound book. The publishing aspect was actually a private enterprise for a while even after the Reporter became an official position, so all the profits from the sales went to the Reporter. Regardless, the Supreme Court, the Library of Congress, and many Washington, D.C., attorney's offices would have had copies of all the volumes of the United States Reports. For example, here are digitized pages from the 1818 Supreme Court session published by Reporter of the Decisions Henry Wheaton, held in the collection of the Library of Congress. The copies more commonly found online are later mid-19th Century reprints but nonetheless, the early volumes of the United States Reports would have been available to Chief Justice Marshall, for him to take to the inauguration. Each of the early volumes are roughly between 100-400 pages long.

Another possibility would be either a volume of the Journal of the House of Representatives of the United States, or the Journal of the Senate of the United States. Article I, Section 5 of the the U.S. Constitution requires:

"Each House shall keep a journal of its proceedings, and from time to time publish the same, excepting such parts as may in their judgment require secrecy; and the yeas and nays of the members of either House, on any question, shall, at the desire of one-fifth of those present, be entered on the journal."

Each house has kept such a journal since the first session of Congress, and you can see the early volumes of the House's journal here. For more information, the Congressional Research Service has recently published a paper entitled "The House Journal: Origin, Purpose, and Approval". Like the United States Report, the early volumes of the Journal of the House, and Journal of the Senate, are generally somewhere between 100 and 400 pages for each session.

If it were some book on Constitutional law, then there would have been several such collections at Adams' and Marshall's disposal. John D. Parson's 1880 Catalog of Law Books lists some of those published by the time of Adams' 1825 inauguration. One of the more famous is Nathan Dane's comprehensive nine-volume A General Abridgment and Digest of American Law, with Occasional Notes and Comments, with the first eight volumes published in 1823-24, and the ninth published in 1829. Each volume is about 700 pages. Dane's contribution to U.S. law has earned him the nickname the "Father of American Jurisprudence".

Another possibility is that John Quincy Adams decided to use a volume of his father John Adams' three-volume A Defence of the Constitutions of Government of the United States of America, published in 1788. While this isn't a book about the U.S. Constitution, it is a book about the theory of American law at the time, and roughly fits the description of "a volume of the laws". Each volume of the book was over 400 pages long when first published.

A less likely book of Constitutional theory to have been used would have been St. George Tucker's View of the Constitution of the United States with Selected Writings, published in 1803, which was over 200 pages in its original publication. Tucker's book was an argument for a Jeffersonian view of the Constitution. It was the first to advance the "Compact Theory" of U.S. law that was at the heart of the "state's rights" view that the Confederates cited in justifying a right to secession in the lead-up to the U.S. Civil War. Neither Adams nor Marshall was a Jeffersonian, so it would have been an unlikely choice to use for Adams' inauguration. But it exemplifies that there was a breadth of available literature on Constitutional law by the time Adams was inaugurated.

Other such books published on Constitutional theory before or near the time of Adams' inauguration include Commentaries on the Constitution of the United States of America by James Wilson (1792), New Views of the Constitution of the United States by John Taylor (1823), and A View of the Constitution of the United States of America by William Rawle (1825).

To answer your second question:

How much Constitutional Law was established in 1825?

If Dane's eight volumes published in 1824-25, comprising more than 6000 pages, don't lead to the answer, then, in 1833, less than a decade after Adams' inauguration, Associate SCOTUS Justice Joseph Story published a book called the Commentaries on the Constitution of the United States. This book was meant to do what William Blackstone's Commentaries on the Laws of England had done to English common law sixty-odd years earlier. That is, it summarized all the most important U.S. Supreme Court decisions, and gave an overview of what U.S. Constitutional law consisted of by that point in U.S. history. It was originally published in two volumes, each over 750 pages long. Much of the text deals with cases issued before Adams' 1825 inauguration. So to answer the question, "How much Constitutional Law was established in 1825?" the answer is, "Quite a bit."

TL;DR: The book could have been quite big, anywhere from 100 pages to 750 pages or more. There was quite a lot of Constitutional law established by 1825.