I use a back pack (momentum 34) and bring my lunch in everyday (same as before I biked). I fit a lunch bag with some sort of salad, with enough room in the pack for a liter of smoothie and a 16oz coffee thermos on either side of the bag. I think the key is having enough expansion to have your lunch setup and still have plenty of room for all of the variables (gym stuff, work clothes in the warmer weather, etc.).

From home refrigerator to work refrigerator takes about an hour and fifteen and I have not had any issues keeping my lunch chilled.