It's been awhile since I've done bags but I do remember some principles... I always used Paton's Merino and if I made one with 75-80 rounds from where the sides or body starts after the bottom is formed, you'll end up with about 10-11" in height after felting. Of course that depends on how long you felt it. They felt faster from round to round and you can figure that if the bag measures 20" when you finish knitting, it'll be about 10" when it's felted. In width, you can figure about 1/3, so if it's 33" when you finish knitting, it'll be about 22-24" when felted. Of course you'll need to know how many sts and rounds the pattern calls for.