Wheels come in various shapes and sizes. Mostly round though.
The general rule to start with: Bigger wheels are faster once at speed and get you to a higher speed in total, smaller wheels are faster to accelerate and allow for more agility.
Let’s get into some more details:
The bigger the wheel the easier it is to maintain your speed. Bigger diameters also have the benefit of rolling over small obstacles, just as small rocks or other dirt that might be in your way, easier. A bigger wheel also leads to a bigger standing height though which makes it harder to stay balanced on them, especially when landing jumps.
Smaller wheels are easier to accelerate but also limit your top speed. The low standing height assures stability, balance and causes less stress on your ankles.
All in all, it’s hard to recommend the perfect size for a skater. Take into consideration your main skating style, environment, ankle stability and skill and be open to test around a bit.