What you linked to are the "Super 8" diffs when UE sells them.
Here are "Super 6" diffs, with bearings and a pinion and everything you need for much much cheaper-
http://www.ebay.com/itm/HoBao-1-8-Hy...item4d1e8e2aef
I have ran very high powered setups with the "super 6" diffs, and in my opinion the extra cost isn't worth it to upgrade to the "super 8" diffs.
To answer your question the bearings are 8x16x5. What you linked to will work fine.