It will all depend massively on your build and the arches and clearances.
Do you have the MX5 donor wheels??? I would start with them.
The cars don't need massively wide wheels for grip or large diameter to exert breaking force.
Too wide and your steering will be unhelpfully heavy and tramline badly.
Too big and you risk smashing bodywork over bumps.
15" on mine, 7J IIRC. Off a mondeo or something.
Also check what tyres are available for the given size, some combinations are expensive