Need info for tire sizes

I wanna get new tires for my rancher 420 it’s 4×4 and Is a manual with a straight axle in the rear I wanna go 27 but I wanna know if I am better with a meatier 26. I know it I get the 27 I’ll loose my top end but I would like to have the ITP mega mayhems but I’m not sure it will turn over in thick mud. I’m trying to stay away from gear reduction as much as possible