I made this bag for my sister for Christmas a few years back. It was so fun to make! The pattern was free on Ravelry and includes charts for all 4 houses. (I really need to make a Ravenclaw one ASAP!) Since I don't know how to sew, I bought fabric for the lining and then gave it to my sister in pieces. (Nothing like a "some-assembly-required" present!)
Here's what it looks like on the inside once she sewed in the lining. This bag definitely combines 2 of my loves - Harry Potter and Fair Isle! It was nice to only have to deal with 2 colors. The bag is actually quite big. Not Hermione's-purse-big, but it could hold a wand and few spell books for sure.
Chels and I getting ready to head to Hogwarts... you can see the size of the purse better here.