Anyway in both cases, crochet or knit, the bag or whatever item, has to be worked much bigger and with thicker needles, two or three sizes higher than usual. The reason is, because the item will shrink a sufficient amount while felting, and because the stitches have to be larger, to give room for shrinking. Usually the piece will shrink about 30 to 40 percent, and it will shrink more in height, than in width with a knitted piece and in reverse with a crocheted piece. This has to be taken into consideration while figuring out the size of any item. To be on the safe side, a patch should be worked and felted to determine the exact amount of shrinking - measuring before and after will tell to the point what needs to be known. The temperature has also to be taken into account, I ruined two bags, which I felted on 60 degrees and the color bled and washed out (I did not felt a swatch before!)
P.S. Any small mistakes made while knitting or crocheting will disappear during felting. An easy way to crochet a simple bag, will be the bucket design. Just crochet a circle in single crochet stitches. The rule here is: the number of stitches you have in your first round, you have to increase in each following round till your circle is as big as you want it to be. If you start with six single crochet stitches, then you will double each one in the next round (12), then every second one (18), every third one (24) and so on. When your circle is big enough just stop increasing and crochet straight up till to your desired height. Remember to make it much bigger, it will shrink a lot. If you use one color you can crochet spiral rounds, just go round and round, without joining them. If you are crocheting stripes, you have to join your rounds with a slip stitch (Kettmasche) to get your stripes butt up. In this case always put a stitch marker in your first stitch. Use a one number smaller hook for the last round and for the handles. Crocheting in the back (or front) loops only will make a less dense surface. Crochet the handles and the bottom always in both loops. Flaps and pockets (which are not on the round) are done with one row in back loops and one row in front loops, alternating.
You can line your bag with cotton fabric, or just leave it as it is, it is quite strong after felting.