size does not mattter... its how you use it that counts
THE factor?... well can my "physio-and-bio-textbooks" count as a 'group' factor? lol As you can see, they can't be separated due to the hyphens, underlining, bolding, and umm... "les guillemets" (quotations mark?) yup, all one thing
Unless, its for an evening/special occasion only bag, a lot of the time when I judge whether a bag is big enough, the deciding factor is whether or not it fits my water bottle.
I tried on the Segur PM once and liked the way it looked, but it didn't fit my water bottle = no buy.
How about everyone else? What does your bag HAVE to be able to fit for you to buy it?