Can't you open the files and look?  Just make it the same size as the ones there (overlay them, if needed).  I don't have Sins on this computer, so I can't answer at the moment.

Actually, I feel like the *.playerPictures (or maybe *.PlayerPortraits) might allow you to define the picture sizes.  It should be easy enough to look up there.

Edit: And don't get impatient if it's only been 2 hours after you posted your question.  Not everyone will know the answer or respond quickly.

Total War must be a heavily modded game, then; this forum is amongst the most active of any modding forum I've been to, and I've modded a lot of games. It seems there just isn't much appeal for modding RTS games like there used to be.

A way to get the dimensions - if you're using photoshop, box select one of the portraits as JSW selected to look at, and copy. Then go to make a new file. Photoshop will set a new file's dimensions to what you currently have in your clipboard. Voila, you have dimensions to work with.