8mm bearings are pretty standard these days if you can fit a #2 pencil through the center of your bearing it is an 8mm. The size of your bearing is not dependent on the wheel it is dependent on your skate axel and most quad new skates come with 8mm and all inline skates require 8mm. If you needed 7mm you would know that you had these specialized skates most likely. 99% of my bearing sales are 8mm so chances are you need 8mm.
BSB Swiss bearings are grease packed. You need to put on 50 or 100 miles before you do a race with them but they last much longer than an oiled bearing. As you skate the grease gets spun up against the shield where the inner race meets the shield and creates a barrier between the contaminants trying to get in and the balls and race.
