tire size?

Ok so bought newer GMC 18' wheels for truck, i noticed that the newer trucks come with a little beefier side wall which i don't want... was wondering what size sidewall i should go with. My truck came with 17' steelies but i forget what the tire size was... should i just figure that out and the run the same but just change the size to 18" instead of 17' or just go with a whole new size?
 
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