anyone here know about tires?
 

I just bought a new 4wd truck and want to change the tires..
I'm used to dealing with sizes that are numbered like 265x75R15, but the ones that are on the truck now are numbered 10.5x30R15 or close.

they're regular road tires and about normal size for this truck, but I want bigger & wider tires for mud & deep snow.

How much bigger can I go without altering the driveshaft or without having to use shock extenders, etc

Also, this decimal sizing got me confused.. what's its size equivalent using the other sizing method?