A general rule of thumb with insulation - either board or glasswool roll - is there is incredibly little waste. Just cut to size and put it in the gaps as best you can. I'm assuming you're going to be covering with a vapour barrier and plasterboard / drywall in any case? It doesn't matter a damn how the insulation looks - as long as it's full coverage with as close to no gaps as possible.

If you get rolls of insulation for the required area just cut to the required width (in your case 22") with a saw - while it's still in the roll. Sheet insulation can be like the worst made jigsaw in the world - who cares? It's going to be hidden in any case. Just slice it all to fit nice and tightly to the studs.