Presumably you have purchased an eBike kit with a in-wheel motor, so you have a wheel but you don't know what size the rim is. What did the kit specifications say about the wheel? Is there a marking on the rim?
An approximately 20mm wide rim is wide enough for mountain bike tires up to 60mm / 2.25 inches in width.
The bicycle wheel diameters in common use are:
- ISO/ETRTO 622 - 622mm rim, known as '700c' on road bike, '29 inch' on MTBs
- ISO/ETRTO 584 - 584mm rim, known as '650b' on road bike, '27.5 inch' on MTBs
- ISO/ETRTO 559 - 559mm rim, known as '26 inch' on MTBs
The ISO/ETRTO number is the diameter of where a cincher tire mounts on the rim. The MTB sizes refer to the nominal diameter of the whole wheel with a tire installed, not the rim diameter. You can measure you wheel rim to figure out what you have. 26 inches is 660mm so I think that measurement is an error.
Bear in mind that bicycle frames are designed for a specific wheel rim diameter. You can't put a 622 wheel in a frame designed for a 559 wheel. You also need to pay attention to the hub width, wheel attachment (nuts, quick release etc) and rim vs disc brake when considering frame compatibility.