Size can be adjusted by the number of beginning chains, as long as its an even number.
Gage does not really matter if you are lining the bag, Hooks H8 or I9 will work just fine, H8 if you are a loose crocheter or I9 if your stitches are a bit tight.
Alternate color work would be 3 pattern rows, 6 rows total of each color before changing to next color.
Or just work the colors as you would like, it would also look lovely in a single shade.