It's worth noting that the medieval warm period lasted about 200-300 years, and substantial growth of a species of reptile would be a matter of hundreds of generations or more. It seems unlikely that the reptiles would change significantly enough in size that people would notice anything out of the ordinary. Also, there would not likely be as robust a biological record that anyone would notice anything out of the ordinary in terms of reptile size.

You might consider asking the first part of your question (about the impact of the medieval warm period on reptile size) on /r/askscience, if you get no responses here.

Do mean higher population or larger in size?