Finding the right size wheel for a electric bike kit
Most ebike kits with hub motors are sized for 26" mountain bike tires and not the larger 700c road/hybrid tire. There are numerous reasons for this:


the smaller the wheel the more torque it can handle as well as deliver 
*conversely, the larger the wheel the larger the hub motor has to be (adding to weight) or it has to be a geared hub motor, adding to complexity. In either case, hub motors for larger wheels are more expensive. 
mountain bike frames are built more solidly and can take the extra weight and abuse
contemporary consumers tend to prefer the mountain bike styling and as a result cheap MTB frames are much more commonly available than 700c frames
if you only want to stock one SKU you'll stick with the most common one.  It doesn't hurt that a 26" wheel is cheaper to ship than a 700c one 


As a result, it's much harder to find an off the shelf 700c rimmed hub motor compared to a 26" one. In most cases, you have to get a hand laced 700c rim on a hub motor. This adds cost and expense and is non-optimal as the motor was likely wound for a smaller wheel and will deliver weaker performance in a 700c. 

Most end users  give up and instead get a cheap 26" mountain bike for their donor bike rather than using their 700c bike. 

And if you have to ask: no, it's not possible to put a 26" wheel in a 700c frame -- not without considerable pain and expense. Seriously, just get a 26" BSO frame or be prepared to pay more for a boutique 700c hub motor. 

Ps. If you're intent on keeping the 700c frame, a better option may be to use a bottom bracket (also known as mid-drive) motor. Bafang is the most common vendor of aftermarket BB motors  and if you google (bafang bottom bracket) you'll find vendors. However, they do tend to be more expensive than hub motors.