To be honest, I was using my left-over canvas material for the bag, so I made the size according to how much I had left.
It's about 1 cm bigger than A4 paper, if that's any help. I can definitely fit all my papers in there with a little extra room. Sides (depth) are 7 cms. Height 30 cms.
If you want something that will fit you well, I'd recommend measuring a bag that you already have. If it's a good size, use the same proportions. If you want it bigger/smaller, you can adjust based on what you'd like the bag to be.