Each one can fit 2 normal sized, 20 oz water bottles into each one along with clean up bags and a few other things. (thoroughlyreviewed.com)
The 48 oz water tank makes a lot of steam so you can clean for a long time - up to 2 hours! (geekwrapped.com)
I forgot to mention that the best gear to have are a 40 oz water bottle, pumping hands free bra, vehicle adapter, and a nursing cover. (exclusivepumping.com)