UNI - where can I see how big my sim's student loan is?

I don't think you can see them anywhere on the UI, the only way to see them is by clicking on the mailbox, and this is just the remaining money your sim still has to pay.

There are fees in the game though, I recently forgot to pay a loan and I got a pop up saying I had to pay the loan + a fee. There, I had the choice to pay or ignore it, the latter which would have probably resulted in an even bigger fee.