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Stirrups are there as REASSURANCE for most riders.
If you aren't comfortable with either one set length or the other, GO IN BETWEEN. biggrin Like, say your "short" stirrup length is oh...from the tip of your middle finger to about a few inched past your elbow, and your Dressage length is from the tip of your middle finger to about your shoulder; You would want to go IN BETWEEN the two. Which would be to about your armpit or a little lower.
THOUGH, you Should NEVER rely on your stirrups to carry you. EVER.
Your legs and thighs and calves do ALL the work for hanging on. Ever do stirrup-less riding or bareback? You don't get stirrups to make you comfy. You get to use your legs. biggrin

The reason for the different stirrup lengths is because different activities are easier or more difficult with them. The reason you keep your stirrups shorter when you're jumping is so that you can get off your horses back and to allow you to close the angle of your hip more efficiently over the fence. However, this tighter knee and hip angle makes it more difficult to sink down in your saddle and sit. With the longer stirrup at your dressage length, you can let your leg drape down your horse's sides and settle into the saddle to sit, but because you have a more open hip angle, you can't fold as well at the hip to get into two-point.

When getting into two-point with longer stirrups, or with posting, your upper-body has to be much more upright than it is when you have your jumping length. Your two-pointis close to vertical position than the actual two-point that you do when you jump. Only raise yourself out of the saddle a few inches, and you hardly incline your body forward at all. It's a similar with posting, keep your upper body straight and don't try to rise as far out of the saddle. Your objective is more to move with the rhythm of the horse than to clear the saddle.
In order to sit the trot with your short stirrups, you need abs. The stronger your abs, the better your sitting trot will be in general. An article I read once said that every step of sitting trot is a sit-up. And that's with dressage length stirrups where you have your long leg to stabilize you. When your stirrups are short, it has to be all in your abs. Someone who rides hunters could give you better advice on sitting the trot correctly in a jumping saddle, as I'm pretty sure I don't do it right. But a lot of it has to do with strength.

If you're just going for a relaxing hack, then put your stirrups at an intermediate length, like Sai said, so that you'll be more comfortable doing whatever. But it is possible to do everything from either length, it just takes and understanding of the repositioning of the body.

i have a comfortable length that i usually do all my normal riding in (schooling, trail rides and what not). i found that with the stirrup lethers i use, that dressage is one hole longer and jumping is 2 shorter. if im riding a horse i dont know or has a history of being wormy or is green, ill often put it one hole shorter than normal because it helps keep a solid base. i hardly ever lengthen my stirrups to dressage length because when i practice for dressage at home, or often just got without. but for me, theres usually a cut. if im going to work on dressge, thats what im doing that day and if im jumping, im jumping. so i have the stirrups ready for the day.

i can keep my heel down and my leg under me easier with a short stirrup. i have a good jumping position but a rather crappy one on the flat or dressage xd (chair seat). i usually dont place well in equitation (well u know, of course above the little girls who cant make their horses go round or the ones that cant stay in the saddle)-ive never won-, but i always do really well in hunt seat (equitation over jumps). haha, i guess im just made to jump.

heh, very true. We have one horse at our school barn (we're not really sure why we have him...) he's a "dressage horse", but mostly because I think he's too crazy for the hunt program. His name is Robbie, but we call him Robbie the llama, or just llama. He's built upside down. Looks kind of like a Dr. Seuss horse. He is physically incapable of bringing his back up because of the way he's built. So I don't sit on him. Ever. I'll sit lightly in the canter, but I refuse to sit his trot, because it's uncomfortable for both of us.

I did 20 minutes of work without stirrups today. I'm not sure if I'll be able to walk tomorrow! xd

Yes. Do posting trot without your stirrups. I did it on Bob, and dear god, his trot is bouncier then ANY Belgians. Lessons without stirrups will make you stronger and not need your stirrups as much.

My position is my legs underneath me and there should be a straight line from your shoulder to your hips to your heels (which should be way down) ^-^ as for two point, I would fold with a flexible back and stay out of the tack until after landing and release some of the reins on the upper mane crest