You can run 32" tires with front coil spacers and a rear add-a-leaf. You will have no gearing issues with the 32's but any bigger will cause the problems articulated so well in the previous post.
Synthetic lubes will help with wear and will keep your MPG close to stock. Unless you go to a 17 or 18 in. wheel and speed rated tires equiv. to approx. 32" dia., you should keep your highway speed below 70 mph or even 65. (for safety and reasonable tire wear)