I'm about to buy an E-bike (brand new, not used) online but the Specs does not say its frame (or wheel) size just that's general adult. One reviewer says this is .
Most adult bikes have 26", 27.5" 29", or 700c wheels, but the part that varies in size is the frame. In order to find which frame is best suited for.
Adults tend to be less cautious in picking the right bike for them but fit also matters for safety and comfort. The right size of bicycle for the right kind of activity will.