Many people say they have used the Sea to Summit dry bags for food bagging when thru hiking the AT, but I see Sea-to-Summit has both a Lightweight dry sack and an ultra-sill dry sack. I was wondering if it makes a difference which one I use? Also, how big are these actually? I have been using an outdoor recreation group yellow bag from Walmart that measures 10.75" x 21" when flattened out. It is about 6.5" diameter x 19" fully loaded which transfers to about 10.33 liters (I think Amazon.com says 8 Liters in their specs). It seems that some people say to use the S2S 13 Liter or the 20 Liter bags. REI shows those bags as 8.5 x 21 for 13 liter and 10x24 for 20 liter. Are the 8.5 and 10 the diameter or half the circumference (bag flattened out)? I'm just trying to compare the sea-to-summit bag sizes to the outdoor recreation group bags that I am already using and which have been fine for me for short trips.