You should post this question on the the Mom's who Pump board too.
I also pump at work (3 times) and once after baby goes to bed. But my DS only takes 3 bottles at daycare (4.5 oz each) - but usually has an extra large feeding when he gets home. (He's there from about 7 to 4.) You do have to watch how much they are giving him at daycare because I heard it is very common for them to try to feed breastfed babies too much. Kelly mom has a good calculator here:
http://www.kellymom.com/bf/pumping/milkcalc.html. Based on 6 feedings a day, the calculator says baby should be eating 3-5 ounces per feeding. So, 6 ounce bottles might be more than he needs. I'd probably go with 5 oz.
I know how hard it can be to keep up sometimes! Hang in there! Having to pump after a long day of work when baby is sleeping and you have many other things to do really stinks!