I would say at that age, you are def. ready to up the prey size. You say you don't understand the girth thing so... (this may or may not work lol)

in your head, take the center of your snake, slice it, and pretend it's like a paper towel tube or something... get a prey that will fit down into that tube perfectly. nothing smaller, nother much bigger.

At that age snake, it can probably take a couple reg. sized mice, but it's hard to tell without actually seeing it