Honestly a fun challenge, go with the smaller size to start off
Honestly, the first bag I bought to make was a really FUN challenge, that made me buy three more bags from the company. My first bag was the Platinum and the largest size, which I would not recommend starting with (if you are a real beginner)- go with the smaller size as your first go around. *IF* you are fairly experienced with sewing, which I would say I am versed in and have made bags before (handmade without patterns or tutorials and mostly with a machine), you could start with the biggest size. My second bag was the smaller Platinum and honestly was such a delight to finish and a little easier (less double stitching and not as hefty that required a thimble, which the larger size I resorted to using a thimble to save my hand strength). My new obsession is sewing up these little bags because they have more meaning when I stitch them up.