When building our projects, tire size is one of the most important considerations. We recently worked with Dan McKeag at Burnsville Off-Road on his build of a 4-cylionder TJ. The choice was made to run 33" Mickey Thompson MTZ tires. After all, lots of fun has been had on 33s over the years. 305/70R16LT tires are about the same size as 35x12.50R15 tires are and provide great flotation. A norrower spare is always better, as it doesn't hang up on steep climbs and weighs less.
The spare size chosen was a 285/70R16LT tire. This is the exact same diameter as the 305, but narrower. It's also a bit lighter. On the carrier, the 285 doesn't stick out much and works great. With the same diameter as the 305s on the ground, there will be no trade-offs if the spare is needed.
When running larger tires, many times there's another size that's narrower, but the same diameter as your flotation tires. It pays to do some homework. The benefit is lighter weight and less overhang on the back of your 4x4.