what size piston should i go with

depends how big your bore is and if there is any damage but if you decide to go with 68.25 that is the last bore you can get on your cylinder if its good id go smaller
 
what size is your piston now? dont bore it all the way out if you dont need to....and if you have to, i would say screw it and buy a BB sleeve and top end.....little more coin, more along the lines of what seems to be popular mod......

if your cylinder sleeve is in good condition...you can just hone it...if you have bad scarring or knicks...you'll need a bore...how big? depends on the flaws.....
 
I am waiting on parts now to rebuild mine. I bought a Weisco 67mm (.040) over. Felt like it was a safe & smart move. More power but not thinning my cylinder wall to much.