I 10 stone and 5'7 and ride a hardy pony 13'2 she wouldn't take much more than that weight

I don't think it's height but weight that's most important x I think you can google the equation x it's a percentage of their weight/ height I think

Hope this helps a little

British Horse Foundation statement

www.bhs.org.uk/~/media/volunteers/resources/events/rider-weight-guidance-and-calculation-2017.ashx?la=en

If the pony were mine I wouldn't let anyone over 9st ride it. Tack weighs a fair bit too.

As well as the equation others have mentioned (the weight of the rider, fully clothed, plus tack shouldn't exceed 20% of the horse's bodyweight), a lot does depend on the ability of the rider and what they are planning to do with the horse.

A beginner will be harder for the pony to carry than someone experienced, because novice riders are less balanced, will tend to 'bounce' on the horses back and are generally less able to carry their own weight. Light work such as gentle hacking or short spells of walking/trotting on a good arena surface is obviously a lot easier than fast work, hunting, intensive schooling for dressage or hacking uphill and over rough ground etc. And of course what a pony could cope with as a one-off or occasional thing (e.g. letting a novice friend have a sit on or having an instructor school a child's pony) is different to what's best for them as their regular rider.

So if you can say a little more about the situation/context people here might be able to advise in more detail. As you mention this is a showjumper you might want to err on the side of caution, regular showjumping at a decent height can be hard on their limbs, esp hocks and back, so I wouldn't want to exacerbate that if your aim is to maximise how long s/he will be competitive for!

Having said that Jamforlunch has a good answer as a starting point, Welsh Cs would typically weigh 300-400 kilo, assuming this is at the lower end of that scale as its a cross, that would give you around 60 kilo or 9.5 stone, so you'd want the rider to be not much more than 9 stone depending on the weight of the saddle and other kit...

That's true, its not just rider weight to think about. I use a synthetic saddle on mine as they will generally weigh less than leather saddles. You can get lightweight stirrups as well (Though I've got Sprenger S4s for my dodgy knees and ankles).

My sister and I had the same 13.2 Connemara cross when we were younger, I think I progressed from her to a 14.2 when I was around 11 so was well under the weight limit but big sister still loved doing games & jumping her till she was 16ish even though she had a horse by then! She was very tall but light and I think she stopped when she was around 9-9.5 stone.

Agree it depends on tack choices, rider ability and pony's physical condition/muscle tone but I think 9.5 stone is reaching the upper limit.

I ride our 13.2hh BRP - she’s around 310kg but finely built with stick legs. Daughter and I are tall but weigh around 50kg each. Pony is fit and well and has never had issues with carrying either of us. Saying that, you can get lightweight riders that sit heavily and vice-versa.... but I wouldn’t let anyone over 8st ride the pony, regardless of how small the were height-wise.

So much depends on how much bone the pony has.

I have a pony of around 11hh, who weighs about 350kg (this is more than the 13.2hh BRP in the answer above), and when she was younger, she used to carry 9 stone no problems - I used to hack her out daily and go for miles. But then she's about as wide as she is tall, and has huge amounts of bone. I have a friend with a Sec B pony who is more than a hand bigger, and he can't carry even that much weight.