ENVO offers multiple sizes in D35 and ST to cater to a wide variety of customers. You can use the following size chart to check which size is best suited for you. While there is an overlap for some models, ultimately it is customer's decision in which size is comfortable for them. There are three main dimensions of a bike which impacts the sizing decision, below you can find out how these dimensions might impact your decision to buy a particular size.

The saddle height should be adjusted as such that while pedaling your leg should not be fully extended at the bottom of the pedal, and your leg should not push against your body on the upswing. ENVO ebikes feature a clamp which you could use to change the height of the saddle 

The effective top tube length impacts your reach, if the top tube length is too long your body will be stretched and that may cause back pain and if the length is too short you may find it difficult to handle the bicycle. ENVO products feature an adjustable handlebar stem which lets the rider do the ergonomic fine-tuning.

This applies only to ENVO D35, as ENVO ST features a step through design. For a comfortable experience, there must always be at least an inch or so space between you and the top tube on your bike when you are standing flat-footed. You can measure your inseam length and check that value against the table provided.