Hello everyone. Noob with a commuting bag question.

I've got a Topeak Explorer rack on it's way, sans bag. I've noticed there are several different types of Topeak bags available, and I'm trying to decide which one would be the right sized one for my needs.

I'm looking for something to carry my slacks (Dockers), shoes (size 10) and shirt for the office. Perhaps a small brown bag sandwich & apple. I'm not going to be grocery shopping or anything, I just need something to transport these items when I bike to work.

Any suggestions as to which one I should try and get?