I'll try to keep this short and sweet but if you have any additional questions about wheel sizes feel free to PM me directly. The factory wheels measure the following...
Front: Height = 12"; Width = 6"; Offset = 4+2
Rear: Height = 12"; Width = 8"; Offset = 6+2
Height - Measured in inches at the outer edge of the wheel; measured straight across (ie. up and down)
Width - Measured in inches at the inner edge of the wheel to the outer edge of the wheel; measured straight across (ie. left and right)
Offset - Measured in inches; 1st number is measured from the inner edge to the back of the face plate; 2nd number is measured from the front of the face plate to the outer edge; 1st number always represents the spacing of the backside of the wheel while the 2nd number always represents the spacing from the front side of the wheel
In full stock form (ie. no aftermarket lift or OEM wheels) you should be able to run up to 27x10x12 tires in the front and 27x12x12 tires in the rear, safely.