Mine is in a 15" cab, and it's a little tight. I have framed cabs, and the interior measurement of the cubby itself is just 12". If your cabs are framed the same way as mine, a 21" cab would have an 18" cubby. This is more than you need, but I don't think it would be bad. The extra space would make it easier to build in the holder and to change rolls.

What if you put the paper towel holder in part, then had the area to the side as a separate little space. If you left 4", you could have another little storage space, maybe for a spare roll of towels. I think I've seen little niches like this on here.

Thanks for the info!