sizing disc brakes

[Caher]

Hi all,
I would like to upgrade my disc brakes (Hope’s Mini Mono being the favourite at the moment) but do not know which size rotor I should get – how is this size dictated? Are there any other measurements that I’d need to know to upgrade?
Currently I have Hayes Mechanicals but these pale into insignificance when compared to my other bikes Magura Hydraulic Rims.

Thanks
Caher