Do you man a thesis? I've never heard a work called a synthesis the same way a work might be called a book or a monograph. A synthesis puts together two ideas to produce a new analysis, and can be applied to all kind of arguments regardless of length.

A book and a monograph are pretty interchangeable, but not entirely. A monograph is literally a single written work, so a book with one author might be a monograph, whereas an edited volume with several authors wouldn't be. A book might be the work of a single author, or it might be the work of several with an editor, or it might be a collection of shorter works by the same author.

A thesis is pretty interchangeable with a dissertation; in my experience these depend on what institution sets the curriculum. Lengths vary on the program; I think my MA was around 60 pages (article length) and my PhD was around 200 (very short, but it passed, so good enough!). They might become an article and a book respectively, if I manage to get around to it.