Table Size/Performance

Is there a way to figure out which file represents which table?

IE: I have a file 21691 and I want to know what table it is.

Also, I've heard that pg splits tables when they get to about 1 gig.

I have a table that could grow to that. It is 700+ megs now. Will
performance/indexes be effected if/when it's size is split.

Thanks
Dennis