Frame size is not just a number on a spec sheet. Getting it wrong is one of the most common reasons a new e-bike feels uncomfortable, and unlike most other specs it cannot be fixed after the fact without buying a different bike.
Height charts are a starting point, not the answer
Every brand publishes a height-to-frame-size chart, but they are not standardised between brands, and they do not account for proportionally long or short legs, arms or torso relative to overall height. Treat a brand's chart as a starting point for which size to test ride, not a guarantee that size will fit.
Standover height
This is the clearance between the top tube and the rider when standing flat-footed over the bike. It matters most for a step-over frame, where you need enough clearance to mount, dismount and put a foot down safely at stops, especially with the extra weight of a battery and motor to manage. A step-through frame (no top tube, or a very low one) removes this concern almost entirely, which is why it is common on commuter and cargo e-bikes.
Reach and riding position
Reach, the horizontal distance from the seat to the handlebars, determines how stretched out or upright the riding position feels. Too long, and the rider ends up hunched with weight on the wrists. Too short, and the bike feels cramped and hard to control at speed. Reach is adjustable within limits by seat position and stem length, but a frame that is fundamentally too big or too small cannot be fully corrected this way.
Why this matters more on an e-bike
An e-bike is heavier than an equivalent non-electric bike, commonly by 8 to 15kg. That extra weight makes a poor-fitting frame harder to manage at low speed, when mounting and dismounting, and when walking the bike, all more often than on a lighter bike.