Can the custom CSS size limit for profiles (5KB) be increased so that it's on par with that of projects?

Probably not, you will have to find a way to reduce your CSS usage. 5kb is already quite a bit. It sounds like you’re embedding files directly into the CSS? Maybe you can use links to resourced hosted elsewhere.

In fact, the limits for game pages might actually be missing the size check!

Thanks, I can certainly do that in one project. The profile, however, doesn't embed files and is a little more complex (lower bound seems to be 4.5K Unicode characters, using one-letter animation names and similarly custom-X classes...)

Anyway, just wanted to add that CSS is typically highly compressible, if bandwidth or page loading speed are of concern (x5 compression ratio using normal gzip, in my case.)