How do i see the size of my trash in iPhoto '11?
It's not that easy since iPhoto uses a database to track the files. When you trash the file, it simply removes the reference to that photo in whatever events and albums it used to be included.

It makes a reference to the original and modified files, so they don't actually get moved into a folder that you can get a quick estimate on the total trash size.

These pointers to a photo are so small that they are a rounding error compared even to the size of a small thumbnail of your photo.

The best way is to just get an estimate how large the typical picture you take is and see how much space a thousand of them take. You'll then be able to know if emptying the trash has any chance on making a difference size wise.