I would highly suggest to stay with the stock tire size.
Why?
because the Kawasaki Engineers engineered that bike with that particular tire size, I know they tested that bike with different size tires and came up with the stock size as the best.
A bigger tire will cut down the power and add an increased load on the clutch and transmission.
A bigger tire may not match the width of the rim.
A bigger tire might be a bear to install, you may have to cut the axle spacer just to get it on the axle, it might rub the side of the swing arm.
Its just too much of a hassle and too much trouble.
The best bias ply cruiser tire is the Pirelli MT66, (Route 66)