Posture on mountain bike long term I have been looking at sizing guides because I'm contemplating ordering a new bike and found out that I'm the totally wrong size for the bike I actually have.

I'm normal proportions and 5 foot 7, but according to the guide here

I should be 6 foot 2.

My issue is that my current bike feels very comfortable to me even though I can't stand flat footed if I stop the bike (just tried it now, I can only just put both feet on the ground at full stretch tip toes, it's a 29 inch wheel mountain bike if that makes any difference. Normally when I stop I just get off the seat and rest on one leg with the other foot on it's pedal. I have tried riding my wife's one which is a 26 inch and has the correct top tube length for me, but it feels horrible and I don't like the handling.

If I was to buy a big framed bike am I setting myself up for health issues in the long term, or should I just go with what's comfortable for me and not worry about it?
Further to the above comments:
Your bike size sounds about right - see here:
Evans size guide
Stand over height is that you can stand over the top tube with your feet flat on the floor and raise the front wheel off the ground. When seated on the saddle you should be able to touch the ground with toes and possibly ball of your foot on one side at least. Look also at having your leg only slightly bent when your pedal is at the bottom of its stroke.

Before ordering a new bike visit a few cycle shops and try different frame sizes and wheel sizes. Mountain bike wheel sizes go from 26" to 29". Some riders prefer a frame on the larger size and some on the smaller size. "Feel" although it cannot be measured is a good indicator of a bike that is right.

Having a bike that is too big may not cause any noticeably negative effects, but handling and efficiency may never be as good as you would want.    

k