When I bike commuted, I could put my work clothes in an Eagle Creek packing folder. That fit inside inside a front pannier bag, along with shoes and lunch. One summer I also would go swimming before work so I also fit bathing suit, small towel, toiletries and breakfast (oatmeal in thermos, cup of yogurt, maybe an apple) in there as well. I would also keep shoes at the office with a small toiletry bag & wipes. I used a front pannier bag b/c I was commuting on a folding bike, and the rack was too low for a regular rear pannier.