You can get 1500 rounds off of a 68/4500, but a 88/4500 would give you a little bit more room. The 88 isn't any longer, just fatter and was an easy transition from a 68.
When I play a corner I usually don't carry anymore then 13 pods, when I'm playing back center I carry 15-16 since I don't have far to run off the break and can get rid of 4-5 pods in the first couple minutes before I need to insert into the middle. If you are playing a back spot, remember as long as you make it to your initial spot you can drop paint if you figure out you don't need it later in the game.
I don't run very fast, but I've yet to get shot out running to my corner this season. You'll probably want to try it out and practice and see what is comfortable for you. I recommend exercising your abs and back so it will be easier to carry more paint if you get up into the 9+ pod range.
For 5-man pick up a 9 pack or one of the larger packs that takes as much space as a 9 pack, but folds when the extra pods are not in use (ER, NarTech/WGP, JT). If I were you I'd get the 88, but a 68 will probably due. I always plan for the future, I knew when I got my last tank that I'd be playing 10-man within 6 months so I picked up the 88. If you are going to play mid then a 68 will do just fine.
Have fun!
-Ron