Capacity.....not sure, but here was our experience this past Christmas. We were in the MK on Dec. 26 of this year (Christmas day it poured most of the day, so we just hotel hopped). We arrived at the park at 9:15 and left at 1:30 and headed to EPCOT. During this 4 hour 15 minute time, we did Buzz, Space Mountain (FP), the new Philarmagic 3D movie (FP), splash Mountain (FP), and Thunder Mountain Railroad (FP). Our last ride was Splash mountain and when we got off, the
MVMCP was in full swing. We walked towards the parade and was met by a sea of humanity. After the parade was over, we did the "shuffle" all the way to basically the hub in front of the Castle. We finally got to the Monorail and headed to EPCOT where the crowds, I thought, were manageable.
So, the week between Christmas/New years seems to be manageable IF you get to any of the parks when it opens (especially the MK) and get out of there around 1:00. Use the fastpass system and you can get on your fav rides.
We arrived on Dec 18 and did all the parks, including Universal and IOA that week with not very big crowds. Great time to go.