This is really a question that might be better aimed at geographers than historians, and you may get more help by asking in another subreddit, but a few comments are pertinent:

First, Lake Chad is extremely shallow - at present the maximum depth is usually around 8m and the average depth in the southern basin which now forms the main stretch of permanent open water is only 2-5m. This makes the lake subject to significant fluctuations in size on a seasonal basis, with size varying depending on monsoon rainfall in the highlands to the south of the lake and on temperatures in the Sahel. These annual variations can be quite dramatic - the length of the current shoreline varies from 500 km to 800 km over the course of a typical year, and they complicate the business of discussing the lake's size over time.

With that said, we know that the lake certainly was much larger than it is now in prehistoric times, in a period when it formed what geographers call lake "Megachad", with a depth of up to 160m and an area of around 400,000 square km, bigger than the Caspian Sea – a size that compares rather dramatically to its current average size of 1,500 square km. This lake ceased to exist around 2000 B.C. We also know that the historic Lake Chad has been showing an overall tendency to contract in size since 1900, and has shrunk dramatically since the early 1960s. The latter impact is partly as a result of the increasing population of the area and increasing demand to draw off water from the rivers that feed into it for irrigation purposes and to feed reservoirs – the Chari (which alone supplies 95% of the water that flows into the lake) is contributing about 75% less water on average per year to the lake than it was 40 years ago, thanks in significant part to the construction of the Maga dam. It is also partly down to climate.

I am not going to get into the issue of climate change, in part as it is well beyond my competence, and in part because – in popular terms at least – the current focus of attention on the shrinking of Lake Chad is a product of its featuring in Al Gore's documentary "An Inconvenient Truth", which first aired in 2006, inside the 20-year period for which there is a moratorium on Askhistorians. However, it does seem worth mentioning that while the consensus certainly is that climate change is an important factor in the shrinkage of Lake Chad, most of the diagrams illustrating the shrinking lake that crop up online take as their starting point the year 1963 – such as this one, from NASA – which, at least according to Lewis and Berry, fell at the end of a short wet period that saw the lake expand to a size that was significantly above its average over the past century or so.

Let's look briefly instead at the history of the lake. It has to be said that anything we know about Lake Chad in antiquity is a produce of scientific research, not written history. Butzer's paper takes the story back to at least 1400, when the lake was part of the territories of what later became known as the Kanem-Bornu empire, and states that over the past 600 years there have been six dry periods which have seen the lake shrink considerably in size, only to recover. The lake is known to have been more or less non-existent around the early 16th century, being then largely marshland, not open water. There's some evidence that the Little Ice Age from c.1570-1850 brought repeated droughts to parts of Africa, which might have impacted on the lake's extent in that period, and in the years before 1914 Tilho recorded significant fluctuations between what he called a "Great Lake Chad" of about 25,000 square km and a Small Lake Chad, roughly corresponding to the lake as it currently exists today. Finally, a fairly detailed and easily accessible description of the lake and its peoples as they existed in the early 1970s can be found in Thor Heyerdahl's book The Ra Expeditions – Lake Chad was one of the places he visited to learn the techniques of constructing rafts from reed bundles.

There's plenty more to be found in scientific sources, but sadly the history of the lake has been relatively little written about, in English at least. From the reading I've done to compile this answer, my impression is that the lake was probably rather larger, on average, in the Roman period that it was in 1900, but that by that stage it had already begun to fluctuate quite dramatically in size, in terms of both seasonal variations and cyclically, over periods of decades and centuries.

Sources

LA Lewis and L Barry, African Environments and Resources

KW Butzer, "Palaeo-environmental perspectives on the Sahel drought of 1968-73", GeoJournal 7 (1983)

Thomas Johnson et al, "Decadal record of climate variability spanning the past 700 yr in the Southern Tropics of East Africa," Geology 29 (2000)

Jean Tilho, "Variations et disparition possible du lac Tchad," Annales de Géographie 37 (1928)

African Countries Mineral Industry Handbook v.4