Well now, that all depends ... It depends on what sort of engine, and it depends how little variation in crank speed you want. So, for example, a single cylinder stationary gas engine used for generating electricity might have two gurt big flywheels to keep the AC frequency steady even during the compression stroke. A multi-cylinder double acting steam engine might not need much flywheel at all.
I did dig out some equations from old books on this topic as I wondered how flywheels scaled, and as far as I can see they don't scale well, not being so effective at small scale, especially if you want to run at slow speed so the gubbins can be viewed working. However, for a scale model, people aren't generally so fussy about the speed varying a little through each revolution (provided it doesn't stop).
If you would like I can root around and dig the equations out, but it might be useful to tell us a little more about your application. I suspect what I have is for single cylinder double-acting steam engines, but memory is a bit vague.
Steve