I don't think it would depend on the size of your horse - a 17 inch saddle is not any bigger on a 18hh horse or any smaller on a 14hh pony. You still want the same neat amount of numnah around the edge of the saddle.
What are "same estimations that everyone uses" as I don't know them and it might help the OP.
For a short backed cob with a 17'' saddle a cob size saddle cloth will be fine.
If in doubt though, go for full size, they usually shrink a bit when you wash them anyway.
Edt so say...If you actually mean a numnah and not the square saddle pads, then I'd go for cob size, nothing worse than seeing a huge numnah that is too big for the shape of the saddle.
I'd have said 'Cob' size TBH. But I have to agree with ironhorse really; so much depends on the brand you're buying and how the numnah's been designed to fit. Some are better than others and some come up bigger/smaller than average.
I have a 17 1/2" dressage saddle and my mare is 16.1hh. I bought a Nuumed numnah size large and it didn't look right. Took it back and swapped it for an XL and it looked so much better. I've also tried the full Mark Todd numnah and that looks too small for my mare.