Maternity size is the same as regular clothes.
If you were a size large in shirts, then you're a size large in mat stuff
This. It will also depend on how much weight you gain later on, and how you carry that weight.
All my mat jeans still fit in the butt and thighs, but the belly panels are starting to get snug. Not really sure what to do about that, other than roll them down :X
ETA, Old Navy offers free returns for online purchases, so... Fear not! I am sending back a shirt this week that made me look like a house. And not the brick kind... ha