If you were to hunt, in numerous states. Would it be advised to buy an atv? If so, then what size would you suggest? Consider added weight of travel, bed of truck or trailor? I'm thinking of 500?! I'm not plowing any fields! Although, since they are built on a 800 frame, could I save weight with a 350/400?? Also, I'm not sure how much I could use one say in Montana, or any other state. These would be "on my own hunts", on public lands. SO, not sure if they are even allowed on Public lands in most states..... anyway thanks for any advice.

I would definitely stay in the 300cc to 500cc range. Not sure how much you can use them on public land hunts. I would check into that before I bought one. Would hate to drag one around and not get to use it.
Someone makes an ATV transport system that works off of your receiver allowing you to still have some bed space. My Polaris 500 takes up nearly the entire bed of my F-250 and I have the 8' bed.
A good pair of boots might better serve you on public land.

many of the states with vast amounts of public land have ATV access. a common thought amongst hunters is atv scare game and atv public land is not as "fruitful". i have a Polaris sportsman 700, if i were to buy an ATV for public land due to its size i would go no bigger than a 500. my polaris 700 gets 15ish MPG and has a 4.75 gal fuel tank or a 70 mile range, add in idling, revving it (for what ever reason) it is probably closer to 60. 70 miles might seem like alot but say your in Wyoming after pronghorn, in relatively flat land you ride 20 miles in the first day, you have 20 miles to back track so 40 total this leaves you with an extra 20 miles (maybe 30) once you get back to your truck. to me this is not a 'large' margin of error say you get lost, start a slow fuel leak, ect,ect. i have heard from people with 400 CC doing 150 miles or more a tank. if it were me i would get a 350/400, powerful enough to get you anywhere yet small enough to not need extra gas tanks(unless your romping on the throttle).

A 500 cc carb'ed will barely get you around in high altitude. If you were to get a 500 I would definitely go FI

I have a 500 Honda Rubicon. enough CC's to work hard and have good power, and flip it in to 4WD and its a play date right there. Mine has a lift kit, big tires, snorkel, exhaust ect... I work my ranch with it and play hard with it. Main thing i would say more then engine size is if you will have a load to carry the majority of the time, get a solid rear axle. it makes a world of difference when hauling things. If you want to play, IRS.

I have the Honda 4 trax 300. It's small enough to get into tight spaces but still has enough power to do all I have asked of it.
I had a Polaris Sportsman 800...big power and EFI. Just to big for what i nas needing it for. If you need to get into tight places dont go for something huge

we have a kawasaki prairie 400 and its a great machine.

does all we need it too, plenty of umph (55mpg top speed) and is comfortable to ride

If you really want to hunt public land you will need a horse/mule, the 4 legged kind. That will allow you to get in the sticks. But don't buy one, there are places that you can rent them from sometimes, plus you have to prove all kinds of shot records.
You can only get so far on the 4 wheeler and in most systems you have to stay on the trails.

All good points, let me add one!
MOST States require you to have the ATV
display an off road permit issued by said
State ---- although not too bad, still an
added expence.

I've had a couple different atv's.

My favorite, and currently the only one I have, is a Suzuki Ozark 250.

It's a bigger frame than most 250's, the only real disadvantage is it only has a 2 1/2 gallon tank.