Review of the Kriega R20 backpack 20 litre storage

Do i go BIG or SMALL?????
Recently purchased the R20. Wanted something not too big as have additional storage with the motorbike.
Purchased from www.sportsbike...
kriega.com
Kriega backpacks range from 4 to 35 litres of storage
cost was £107 plus tax
Looks like its going to be a good size.
BUT if you have no other way to carry your stuff then go BIGGER!!!