Perhaps consider breeding a WB mare to a larger welsh stallion, like a section B. Most likely a 16+ hand mare bred to such a pony would give you waht you are looking for. By using the WB as the mare component you stand a better chance of size as womb allows the fola to develop larger than if the mare were the welsh half.
I've contemplated breeding my Trakehner mare to a Welsh, so I think it's a lovely idea. She's only 15hh, though, and her daughters so far have been about her size, so I might actually stand a chance at getting a pony out of her.
Don't think I'll actually breed her again, though. Having too much fun riding her, and I'm keeping her 2007 daughter, so the barn is full!
Actually the German Riding ponies evolved from crossing the smaller WB mares with Welsh and Anglo/Arabs. If you look at the pedigrees of many top GRP's you will see some Welsh several generations back.
We've bred a small WB mare to Welsh and the results were very good. We also bred her to 2 different GRP's - these all went "hony" size but are super under saddle and doing very well with their owners in competition.
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We are doing the WB X Welsh cross for a 2011 foal. There was a large pony that showed here on the Ontario circuit last year that I absolutely loved! When I had the opportunity to purchase a smaller Cdn WB (Dutch WB X Trak) I jumped on it. http://www.allbreedpedigree.com/maeve
She was bred to Llanarth Senator, but I don't think she caught. We are going to re-breed her to our Morton's Harkin if she didn't catch to Senator.
I also have two other mares that will be joining in the WB X Welsh cross in a few years. The first is our imported Oldenburg WB mare who is still currently showing. She is by Corradino out of Wachtel by Wenzel I http://www.allbreedpedigree.com/century10