Truly there are sizes available, but the only difference between sizes is the height of the handlebar, which are approx 6cm apart.
Rider weight does concern the stiffness of the springs. However, we'd recommend going with the ones that are for your weight. Softer springs might make the Halfbike a bit too playful for a beginner, while harder ones can make turning close to impossible.
When ordering a Halfbike 3 you're presented with 3 options of gear sprockets to chose from, depending on the terrain that you ride regularly. If your city or daily commute would be on mostly flat terrain you should go with the 14T sprocket. If it's a mix of flat ground and gentle slopes you should go with 15T. If it's a more hilly area you should go with the 17T sprocket. The changes are quite subtle, but they do help a little bit on said terrain