Without seeing the pony it's kind of hard to say as even newfies can vary in build and type these days,I would say anywhere from 10-12 stone??
Depends on the type of work it would be doing as well.
If it's any help I have a appy x native of a similar height who I would say is of substantial build but not overly stocky,and he carries me at just under 11 stone without any obvious issues.I would consider that about his top end of weight carrying ability though for proper work on regular basis.Not to say he couldn't carry more as am sure he could,but I wouldn't want him to ideally.
Apologies if that about as much help as a chocolate fireguard lol
My 14.1 nf is chunky build im 11stone and my instructor 12 1/2 stone n carries us perfectly but ive had to argue she is a nf before as everyone says she looks welsh d.....
Weight wise they can carry a fair amount as others have said but with a 14.1 anything you have to take into consideration the length of the saddle as well otherwise you can end up with some pretty terrible back problems, the length of your thigh is important to consider as it will determine how far back in the saddle you sit which will impact on the effectiveness of the weight distributing panels!
Agree with Emmaln, weight wise 11/12 stone I would say for a fairly well built NF, but as long as the saddle fits well, isn't too long, and your not sat right on the back of it