you should also be able to use Magura rotors as they are the same thickness as shimano ones. any other make and you will need to check the thickness of the rotor. also bear in mind that the braking surface will need to be the same size...if you use a rotor with a smaller surface then you may end up wearing a groove into the pads and eventually you may get a situation where the pads wont bite onto the rotor.
regarding rotor size you can fit any size of rotor with the correct adapter. just check that you dont void any warranty on the fork by using something bigger than is recommended.
i presume when you say that your current ones are post mount, they are for the front brake, in which case you'll need a post to post adapter to take your new brake from its standard 160mm up to 180mm or 203mm.
if its for the rear it will be the same but normally most rear brakes still use the international standard in which case you will need the same adapters, but if your rear brake is post mount and the frame is IS then you will need a post to IS mount. the same theory applies for going from 160mm to 180mm or 203mm.
most brake manufacturers do sell the adapters as do a number of other component makers such as A2Z. i've used hope in the past and they are good.
the best thing would be to pop down to your LBS and see what they suggest or have for you.