Editing party: It was fixed!
Roblox used to remember what window size I left it at when I left the last game I played.
However, now it’s making the window go fullscreen every time I join a game. Fullscreen lags the crap out of my computer, and it’s being really annoying right now. I have to manually resize the window to be a lot smaller.
EDIT: This also seems to be happening to others in the “Help (Technical Support and Account Issues)” Forum, and it seems to apply to Roblox Studio as well as the game client.
Another EDIT: I should also probably mention that I’m using Windows 7.