Anyone here use the Ortlieb Front Roller bags on their back rack for commuting (I want to make sure they'll work on the back rack)?

I can't find anyplace to put my hands on a set to see how large/small they are and I want something that will carry your average commuter clothing items, laptop and a few misc work folders and wonder if these are a good size for most folks. I'm thinking the back rollers are overkill for me for basic commuting and I'll probably never go on long rides where I need to pack that much stuff.