Ask any experienced EUC rider what the most important spec is, and wheel size will come up immediately. It affects the ride feel more fundamentally than motor wattage, battery capacity, or even suspension.
14 inch: Learning and lightweight
Small, light, agile. Perfect for learning and for riders who value portability. Max out around 20 mph. The Airwheel X3 lives here.
16 inch: The performance sweet spot
The most popular category for good reason. Fast enough for serious riding (25-45 mph), light enough to be manageable, agile enough for city riding. The InMotion V12, KingSong 16X, and countless classics live here.
18 inch: Commuter king
Smoother over rough terrain, slightly less agile but more planted at speed. The Ninebot Z10's cult status came from its fat 18" tyre. Great for mixed urban/suburban commuting.
20 inch and above: Touring behemoths
The KingSong S20, Veteran Sherman, and Begode Master live here. Superb at highway speeds, phenomenal over rough terrain, outstanding range, but heavy and less nimble in tight spaces. For experienced riders only.