I'm guessing I'm going to need a new bag/diaper bag since DS1's stuff pretty much takes up most of the bag now....and def was filled when he was a baby.  Right now, my main compartment is just big enough for about 2 diaps, small wipes, DS's sippy and a snack cup.  
I hate carrying bags......how big of bag am I going to need?   
Re: How big of bag is needed for 2u2?
Depends on your lifestyle. I'm a big bag carrying type of person in general, so I don't mind big bags.
I would wait until your second gets here and see what your needs are and then purchase a bag.