over my various commutes I have used different sized bags. I used to use an oversize under the seatbag with a lot of success.

The key for me, espeicially if was a longer commute and I put on bike clothes, was leaving a pair of shoes at the office shoes take a huge about of volume.

Now I can commute in my work clothes (short commute) and use wald folding baskets....but I also usually have my son's back pack as well as my stuff (sometimes laptop)...and I always carry bungee cords......kind of a bicycle baleing wire thing