Is there actually a way to figure out how large Kanto is and how long it would actually take to get through all the gyms?
>Hike to Mt. Moon
>Buy Magikarp from shady guy
>Grind in nearby grass for a little while, meanwhile using Pewter as a rest point
>Literally plow through everything on the back of an angry hovering sea serpent
>>32930248
https://en.wikipedia.org/wiki/Kant%C5%8D_region
>32,423.90 km2
Outside of manually counting the blocks, no.
However if I had to guess I'd say that the Kanto area was something like 2000x1000 blocks, and you could probably say a block is between a yard and a meter.
If you're talking about how big the actual real life Kanto is, there is no canon answer to that, just fan maps.