One of the biggest frustrations of running a rescue is trying to educate people about the minimum size of run that will provide a suitable exercise area for bunnies. We ask for a minimum run size of 6ft by 10ft. The RWAF currently say 8ft by 6ft. Pet shops generally say "any size that gets you to part with your cash before you leave the shop".
We also ask for a minimum of 3ft high to allow comfortable headroom for binkying, though we suggest higher ones are better to allow you to sit in the run and interact with the bunnies.
There's another reason why the taller ones are better...
The extra height might keep the local undesirables off your roof!