Need help buying the right piston

It wise to rebore and install an oversize piston when a relatively catastrophic failure like the one you describe occurs, since the smashed piston bits and flailing Conrad end tend to damage the bore surface. As long as your bike does not have a chrome, nikasil or similar plating inside its cylinder it should be easy to rebore. In this case it would have an iron liner (tubular sleeve) cast into the aluminium cylinder. And the manufacturer (or aftermarket, eg Wiseco as someone suggested) will have oversize pistons available, usually in 0.50mm steps.

If you have a plated cylinder, it is a much easier but more expensive fix - just replace the whole top end ie piston & rings, cylinder, rod bearing, gaskets and seals. Check your power valve and reed block and br sure to check the crankshaft for damage and/or excessive runout, before dropping all that loot on a new top end.