As far as wheel sizes go I feel confident with 8" tires. True a 12" wheel spins 30% less than an 8". Typically 12" tires are wider than 8" tires, which means more rubber is on the road, which means more traction.

In mechanical design we tend to deal with cyclical loads. Funny things happen when a load occurs over and over again. Think about bending a paper clip, you can do it once or twice but if you do it over and over again eventually it breaks. The goal of mechanical engineering is to design paper clips that don’t break. Bearings are designed to have an “infinite” life (on paper any way). On the box it never says “replace after 1 million revolutions.” Industry standards such as SAE are set up to make sure that machine parts don’t have a finite lifespan.

What is more important than wheel size is that your hubs are well maintained. Well maintained bearings with an 8" wheel will last for years and years and miles and miles. Poorly maintained bearings regardless of wheel size can be extremely dangerous. It is all about lubrication and maintenance.

A friend of mine was towing his Boston whaler from Tiburon (just across the Golden Gate from SF) down to Monterey. He didn’t know about bearings or how to pack his bearing buddies. Just south of San Francisco he noticed his trailer was smoking and pulled over right away. A bearing had seized and when he jacked his trailer up the wheel fell right off. The heat had softened his axel and the seized bearing has sheared the wheel right off. I guess the moral of the story is to check your bearing buddies regularly.