In reply to discosucks:
> Now i often seen 'larger' people at crags or walls with there harness below there protruding belly and I know this cant be right and dangerous if they ended up upside down .
That's actually fine if their body weight distribution allows them to tighten the harness below a roll of belly but at a point which is above and narrower than their hips. Which will be true for some fat people but not others -- people come in very different shapes, and some are more hourglass-y than others.
> I have a big chest and i narrow at the centre but don't really have large hips to hold the harness up lol
Do you have a waist which is at all narrower than your hipbones?
If so, tighten the harness at that point so it's also narrower than your hipbones, and gear will make it sag but can't pull it down over your hips (and also you can't fall out if you get turned upside down).
"Large hips" and squats aren't necessary because it's not your glutes that should be keeping it up, it's your iliac crests. *g*
If the waist belt can't be tightened that much, then it's too big.
I have a friend who's extremely V-shaped with very narrow hips, and once (having forgotten to bring his harness to the wall) he had to resort to renting a "children's" harness to get one that fitted.
If you don't have a waist point which is narrower than your hipbones (for whatever reason), then you'd need to think about a full body harness (or adding a chest harness to your existing harness).