Once upon a time the rule was you measured the horse's cannon bone just below the knee in inches and doubled the number to get the weight in stones that the horse should be able to carry ...for example 8 inches of bone would equal 16 stone of a rider.
Welsh cobs don't come along with tags stating "gross payload rating".
A fit 10yo 15h Welsh Cob stallion with Brenin Gwalia in its pedigree in good condition fed well will carry any weight of man over any distance. But don't ask it to jump 4ft fences after going 20 miles. A pretty silky 14h gelding with Derwen in its pedigree is little more than a tall Section C and was bred to carry a light young woman and to perform at horse shows.
If you keep your Welshie well exercised, in good condition; if it has been schooled properly and you are a sympathetic competent rider, then you can weigh 200lbs or more and the horse will carry you for miles over uneven terrain uphill and downhill - all day.
But a certified guaranteed performance won't ever be offered by the breeder.
Welshies notoriously vary significantly in conformation. Some families are pretty, some are hairy load carriers. But you the owner rider should be able to feel if your horse is up to weight and if you feel that he is winded or sluggish or weary, then you get off and walk.
The breed emanates from the Welsh mountains and are known to be tough little critters who have shortish legs because they keep having to climb up and down hillsides. They live out, are do-gooders and can be quite fiery. They will carry any weight of human for a short distance but they will break through over loading and over work just like any other horse.
Just listen to and feel for your horse. He is not a machine.
My trainers rule of thumb for horse comfort was if 6 times your weight was the horse's weight (or the horses weight was more) you were fine. This American pounds, btw...I don't know the conversion to stone.
Example: I weigh 110, so I can comfortably (for me and the horse) ride a horse 660lbs or over.