You can go up to 26" without a lift, but I think 27" will rub.
Width would depend on which wheels you have and their offset, but you can typically go 1" wider without issues. Just measure the clearance between the tire and the nearest point of potential contact. You'll need a little more than 1/2" of clearance (3/4" would be comforting) to go to a 1" wider tire.