Large wheels> 85mm roll well over unevenness and keep high speeds. Smaller wheels accelerate faster and you have more control over roller skates with small wheels.
Soft wheels are quickly worn, but have a good grip on the surface and usually more comfortable to drive over uneven surfaces. Hard wheels have a long life and have a lower grip on the surface. You are compromised with the grip when the surface is even.
You can read more in our rollerskate guide found on the homepage.