You need to look at the upper ball joint, there isn't much clearance there. Therefor you need to keep the tire height about the same as stock. Stock is 29.57 inches. I currently run Michelin Cross Terrains at 275-55-18 with an overall height of 29.90 inches and I don't have much room to spare but there are no issues and of course as the tire wear the gap gets bigger.
Generally 30.5", but many tires are larger or smaller than their "nominal" size would suggest. No substitute for finding a tire shop that will do a test fit. Definitely go under there and look at the relationship of the tire edge to the upper ball joint mount to see the issue.