I was looking for a front motor kit for a 700c Schwinn bike I have and they have this chart. They show 27.5 and 28". I was thinking it's 28" but not too sure. I attached an image so you can see what I mean.
Continental lists their 700c tires as both 28" and 29". I use their Top Contact II tires on my 700c 2020 Giant Roam 3.
700c is also known as 622mm, and this is marked on my rims and on the tires as well. So when I bought the new tires, I shopped for 622.
But Continental also has a different tire, a 622, that is listed as 29"
It's confusing because it seems there's not a single method of indicating tire size, there's older and newer methods.
If I actually measure my mounted tire, it's just about 28.5"
Here's two screen shots from the Continental site. One shows 28" tires - this is for my Top Contact II tires. The ETRTO diameter values for these tires are all 622, with different widths. (I use 47mm.) The other shot is for their Double Fighter III tire, also 622 diameter, but a bit wider at 50mm. However this is listed as a 29" tire. I can only guess that a wider tire will actually inflate to a slightly larger size - making it closer to 29".
I would suggest contacting the vendor just to make sure which kit is compatible with a 700c frame.
Or...go with a mid-drive motor!