Wheel sizes can vary on electric bikes dependant on the type of bike and frame size. Just to confuse you even more some wheel sizes are referred to differently but are actually the same.
Here is a handy guide to explain what the differences are and some of the pros and cons of each.
These are typically found on folding, cargo or compact electric bikes. On folders and compacts they will reduce the footprint and weight of the bike when folded. For cargo bikes the smaller wheel diameter is used for strength so it can carry heavy loads. A bike with smaller wheels can only really be used on hard surfaces.
This used to be the default wheel size for MTB and Leisure bikes but 26" wheels are quite rare now and usually associated with the smallest frame sizes. A smaller diameter wheel will effectively lower the bike and therefore may be considered for very short riders although manufacturers are gradually phasing these out now to concentrate on 650b or 27.5"wheels instead.
27.5" and 650b are the same wheel size and are now commonly used in place of 26" wheels. They are usually found on eMTB's and some leisure and trekking bikes. It is a shorter diameter wheel than the most common 700c and 29" and can also be found on smaller frame sizes (where a larger size has a 29” wheel). With a 27.5 wheel the bike will have a shorter wheelbase and be more nimble on tight and twisty trails.
29" and 700c are the same wheel size. 29" is associated with MTB's and 700c leisure, road and gravel ebikes. Although they have the same diameter the main difference is rim width. MTB's use a wider rim so they can accomodate a much bigger volume/width tyre. Electric bikes with bigger diameter wheels have a longer wheelbase and will not be as nimble as their smaller wheeled counterparts but they will roll better and soak up bumps on trails better. On road/gravel bikes you don't really get a choice. It's almost exclusively 700c.