Whilst I never go overly big, usually either enough for science equipment and a sight-seeing tour through the cameras of a rover or just enough for a buggy that isn't going to weigh me down on the launchpad because I'm still just trying to explore and plant a flag on almost all celestial bodies in the system. I find that planning rovers for Gilly-like conditions is best in any situation so you'll have little as possible chance for it to tip, make sure its base is wide and possibly long, rotate the wheels to almost 45 degrees (so shift+eight twists to 40 degrees as it doesn't register wheels at 45 or beyond I find) to make the profile as low as you can without causing anything on the bottom to hit the ground. When I'm on Minmus, I take advantage of its superbly flat and frozen ground and put on the airplane wheels that have no maximum speed and let ion engines take me to where I want to go if I need to go quickly (getting to orbit this way is certainly fun). For rough terrain, I'm still puzzling how to make something to my aesthetic taste but if you take the structural girders and plonk wheels on them, you'll find they can give better suspension when on places like the Mün because they bend to the landscape from where they're joined to the body of your rover, they probably will do worse and worse when you use them on large enough rigs such as Kethane would likely bring. Then if you're looking to go up a hill, the Kerbal Attachment System mod will likely give something to get up with if you're struggling to get a Kethane miner up although I wouldn't rely on it, moreso something to lift you up and over the obstacle as you can always mine more rocket fuel with Kethane to replace the amount spent, right?