I recently purchased the 21x7-8 tire and found it to barely squeeze into the frame width wise on the rear... Actually there is a ton of space left over for the frame but the rear sprocket is the limiting factor. This tire inflates to 7.75 inches wide and leaves only an 3/16 of an inch clearance to the tire.
https://www.amazon.com/gp/product/B01MRLBXVI/ref=ppx_yo_dt_b_asin_image_o00_s00?ie=UTF8&psc=1
On the front there is absolutely no problem and we could go slightly bigger if I could find a tire slightly bigger In this picture the front tire is the original OEM tire and the rear is the new SUNF 21x7-8 tire
https://www.amazon.com/gp/product/B01MRLBXVI/ref=ppx_yo_dt_b_asin_image_o00_s00?ie=UTF8&psc=1
On the front there is absolutely no problem and we could go slightly bigger if I could find a tire slightly bigger In this picture the front tire is the original OEM tire and the rear is the new SUNF 21x7-8 tire