The dimensions of the 2 liter dry sack are 5.1 x 11.4 in.
You could probably fit the King Solomon in the 20L Dry Sack, but since the roll-top takes some of your volume, I'd play it safe and go with the 35L. You can still get it snug by rolling it an extra couple times.
When you go to order, you have to use the drop down that says "Choose an Option" and that is when you will choose the color you want.