16 stone is not that heavy. Saying you need a shire is a little offending, i have a 16.1 TB who when i was 16 stone could easily carry me and do good work with me. After a fall i put more weight on but didn't ride, and i'm now slowly getting back to a weight i will feel comfotable getting on him again. Riding will make you loose weight anyway so go for a horse that you feel confident on and only worry about your size if the horse appears to struggle. The ones i wouldnt suggest are the very fine ones like arabs or the like, but my TB is a hunter type.

this sugject is close to my heart too, I used to ride a coloured cob for a friend of mine as she bought another horse and couldnt ride her cob for a while as she wanted to get her new one right, after a while she told me that i couldnt ride her cob anymore because i was too fat, I'm 14st and 5ft 4.. her boyfriend now rides him, who by the way is much heavier than me. it still upset me when i think about it but I now ride a 16.2 throughbred, he's the sweetest thing ever, a real gentleman horse, loves going to the gallops... its true, its down to the individual animal, their movement, they'll tell you if they are unhappy!!

It's not just your weight, but also how you hold yourself in the saddle.  People who are unbalanced usually cause more damage to a horses mouth and back than any other rider.

Also, think about getting a horse well balanced in himself.  It doesn't have to be 18hh, but more square horse (short back, wide shoulders and hips, and big bone) may help you feel more secure.  The best breeds to go for are generally cobs or heavyweight crosses.

I don't think you can ever be too heavy to own a horse, so long as you are physically fit enough to ride properly and choose the horse wisely.

Good luck.