Head gasket

I’m building up my short block mx5 mk1 with forged internals, I’ve also had the bores oversized from 78mm to 79mm. When looking at head gaskets how much bigger do the gasket bores have to be compared to the cylinders?

Also am I correct in assuming that the thickness of the gasket will affect the compression ratio? My build is a low compression, supercharged engine. Would a thinner gasket be more beneficial in this set up?

Thanks, any advice would be appreciated.

As long as the bore diameter of the gasket is at least as large as the bore diameter in the block then you're good to go. Generally I aim for the bore diameter of the gasket being the bore diameter of the block +0.0-1.0mm. i.e if your bore is 79 mm I'd be looking for a gasket with a 79-80 mm diameter.