There is a nice variety of tank sizes that you can work with if you're looking for a larger tank. It's not all that odd to see 88/45 tanks with the woodsball/scenario crowd. Plus it depends what your price range is for said tank, where you fill it (a local pro shop, scuba store, field, etc.), and if care if it's new or used.
A 68/45 will significantly increase the amount of play time you will have in the woods. I play mostly psp style fields so I use a 68/45 which I love. However one of my friends has a ninja dura 77/45 which isn't much longer that he uses in the woods. I used it once and it wasn't that much bigger. So if you need play scenarios where you're on the field for hours at a time and you have the cash, drop the money for a 77/45 because it feels significantly smaller than an 88 but will be able to hold a LOT more air than your 68/30
__________________ "You are right, my penis simply will not accept anything smaller than 68 caliber."