2008 gmc 1500- upgrade

I just purchased an 08 sierra crew cab SLE. I got a 2.5"(front), 1.5"(rear)leveling kit ordered for it. Just wondering what would be the best tire and size to go with. stay with the 17" or could I go to the 18"? New at this so have some patience please people.
I dont want to go with big old tires cause i do a bit of highway driving but alot of dirt road. I was thinking the BF Good rich Mud terrains 265-70-17..
Any thoughts or ideas of other upgrades would be greatly appreciated. I'm going to post a pic of the truck as it is stock.
thanks again and its glad to be apart of this site.
Cheers!!  

Search the tires you're wanting to buy on any search site to garner reviews on your tires.  The actual diameter of the tire doesn't have much effect as does the overall diameter.  Do not go to a 22" wheel and a rubber band for a tire(low sidewall number), as your ride quality will be terrible. Use an LT rated tire, it will make a huge difference in handling vs a P tire.  Stiffer sidewall, slightly stiffer ride that you will probably never even notice, but will handle much better, especially when towing.

As for other upgrades, look at airbags.  Most for 1/2 tons are only 1000# Firestone setups, look at Hellwig 6012 kit(5,000# assist).  I think though for 07/up GM trucks, there is a new 9" wide bag that retracts when not needed, but personally, I think it ruins the point to retract.  
I just put Hellwig's 6012 kit on my 05 and the difference with 20# of pressure when unloaded vs stock is amazing.  It handles better, not as bouncy on junk roads, and rides sooooo much smoother.  Throw a trailer on it, pump to 60# and it's level, plus the ride and control are worlds apart vs stock.  Seriously, it's the best thing I've ever done to a truck in many, many miles.  Maybe my springs were sagging a bit due to my mileage on truck and not very happy with hauling loads any more, but regardless, I am beyond thrilled with installing the bags. Bought from Ebay for the best price, installed myself.  Get compressor kit, it will make life much easier.