We have no idea what length the Welsh bows were before and if they were influenced by the numerous men who served in the English/Anglo Norman armies. If however Gerald is to be believed as to their power and efficacy, it is highly likely that they were the same sort of length as that used by the English, short heavy bows are much more likely to break than long heavey ones and one has to assume that the Welsh knew what they were doing when it came to weapons for they are reputed to have been a very berligerant people, as ready to fight each other as they were ready to fight the English/Normans and it has to be said they gave the English a run for their money over quite a few years. In addition Gerald in his remarks on Welsh bows made no mention of their length, instead remarking on their (in his view), crude manufacture and that they were made out of dwarf , presumably Wych, Elm. Which leads to the conclusion that their length was similar to the lengths of the bows he was use to seeing in Norman England.
As for making ther bow famous, here I have to agree with adb the English certainly did do that.
For those interested in the shapes of the ancient bows found throughout northern Europe I refer you to Clarke's paper on Neolithic bows, it is still around on the internet, Gad Rausing's "The Bow", still available from secondhand bookshops or as a print of the second edition from the Simon Archery Foundation, University of Manchester and even TTBB vol 4 where there is a, non exhaustive, list of bow finds with, in most cases, a brief description of their shape.
Sigurd,
One book you may like to read, which amongst other things shows the falacy of the the Welsh invention of the long-bow, is Strickland and Hardy's "The Great War bow" which must have been reprinted as I have noticed it for sale recently on Amazon by Amazon, at half the price I had to pay for my secondhand copy a few years ago. I cannot recommend it enough for those interested in the History of the Warbow
Craig