Firstly, Im sure DAMO is talking about the height/length of the sleeve. as you will have XXinches up/down adjustment.
Secondly, the ID of the sleeve should be as close as possible, if not exactly the same as the OD of your strut (without being smaller) for example:
If your strut has an outside diameter of 2.5 inches then your sleeve should have an Inside diameter of 2.5 inches also.
But then again you might be confusing the coil-over adjustment threaded 'sleeve' with the actual coil 'spring'
If you do indeed mean spring, then there are no great differences other than diameter. But their fitment will be dictated by the diameter of your spring platform.
As for differences in rate/progressiveness this is because all coil springs have a different use and thier use may require a pre-determined rate. Of the shelf car springs are ususlly a hit and miss affair, which unless your suspension setup is right for that particular pre-designed spring you may as well not have wasted your cash.
If you are buying them for you'r car, get some made to suit you'r required rate and height.