I agree with oldbones. Sometimes it's best to start out on something a little more tame like a 250 4-stroker if you're doing the following riding: rocky/muddy conditions, hill climbs, tight-single track, thick woods and trails, etc. They are certainly easier to pickup after dropping. They are easier to throw around=less tired at the end of the day. To address your stated conditions, open roads, sandy beaches, I think you could easily handle the 400's and larger. Just be aware that a properly tuned 400 or larger 4-stroke when gassed in the first 3 gears will turn-over backwars.