16 bearings are needed for quad skates 8 wheels x 2 bearings are needed for each wheel in quads and inlines  
Don't forget, inline wheels also require spacers
 
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.
 
DETAILS :
Legendary Original Bones® Swiss Bearings are the optimum combination of Bones Skate Rated™ design and Swiss precision manufacturing creating the standard by which all other bearings are measured. For more than 36 years preferred by the world's top professional skaters. Proven, professional competition-grade speed and durability make Bones Swiss normally last several times as long as ordinary ABEC rated bearings. Made in Switzerland.  
 
FEATURES:
Single, removable, non-contact rubber shield allows easy cleaning and low friction.
A high-speed nylon ball retainer provides added strength and speed.
Pre-lubricated with Speed Cream racing lubricant
Skate Rated; clearances, tolerances, materials, and lubricant provide the best performance and durability possible.
Best selling high-end skate bearing in the U.S.A.  
Bones Swiss Bearings have the best reputation in the skateboard industry and have had this honor since 1981, due to their performance and quality.
Everyone who rides the Swiss knows how fast they are and how long they last. Testimonials reveal them lasting several times longer than low-cost bearings.
Skaters who use Bones Swiss do so because they want the very best equipment they can get and know the difference between ordinary and exceptional.
Our team is a who’s who of top skaters and we are proud to be associated with each one of them.