What is the right size ebike for me? 

The basic sizing method of bicycles, will look for too things: 

1. You should be able to get the seat high enough so that your knees have a slight bend when pedaling, if you can't get your knee is too bent, the bike is too small for you
2. If the bike has a top bar, you should be able to stand over the bar without touching it, like shown in the picture below: 

Seat Height

The correct seat height should have your knee slightly bent, like shown in the picture below. A seat that is too low can cause knee issue as they repeatedly bend excessively (the lower the seat, the more it bends). A seat that is too high will risk hyper extending the knee