12" or 14" rims?

I would say that with the larger rim, you will have a smaller sidewall on the tire which will give you more of a harsh ride.

Some people prefer a more harsh "sportier" ride, so the 14s may be just up your alley. Either way, the Bighorns ride great, so harsh ride even with 14s probably is a non-issue.

If it were any other tire than the Bighorn, I personally would choose the 12" rims.

There was a guy running Bighorns on a Rhino 660 with 14" rims on this site, or maybe it was the www.atvnation.com site, and he had nice pics and a nice writeup. Do a search on both sites. I know it's out there. That may help you to decide what's right for you.

Less sidewall = better. less chances of rocks stabbing my tire. The trails around here are known for blowing out stock tires fast because the Mountain has some pretty harsh rocks.

Besides my ATV has an adjustable suspension, so i can compensate for the harsh ride

IMO i rather replace tires than replacing wheels. a 14" wheel just means that there is more wheel to bang up. 14" wheels are fine for regular riding but if there is alot of rocks i would just get 12". as far as the size of the tire, i would go with a 26" tire since your bike is stock performance wise.
-Aaron

Well bighorn tires are about an inch bigger than what they say is. The trails aren't 100% rocks, but once I get up to a certain point in the mountains it gets a little rocky, and I do some riding through/along a creek bed so you can obviously tell how many rocks there is there. I'm still undecided really, a 14" tire would look very nice and I don't think having a rock come up and hit the rim would be too much of an issue since its not like I drive very fast on the rocks. Performance wise I wouldn't want to get much into it but I plan on modifying it a little bit, like just bolt on/replacement parts.

I have the 14" ss108's with 26" bighorns, love the ride and the look, and despite what I have heard from some, they arent that bad in the mud. because there is some mud that dosent care what tire you have, you just aint gonna make it... lol