Hi There,
In most major UK Loch Style Competitions the maximum length of a hook is 5/8 of an inch which equated to an old standard shank size 10.
In addition the maximum overall length of a fly including the hook is 15/16th of an inch.
This might seem an odd size but it is the hook length plus 50%.
We are also not allowed to use flies that are additionally weighted...so no metal beads or lead wire or excessive copper wire. I think this rule was brought in for safety reasons because there is usually one person casting "through the boat"
Our first rule in the book is that the competition should be carried out in the spirit and tradition of the sport.
That I think is where the fly size rule comes from.
The English rules which are broadly similar to the rest of the UK can be found here.
http://www.ceff.org.uk/ceffrules.htmlLoch Style Competitions have been held in Scotland for well over a hundred years and the Home Internationals go back to around 1928.
A book on the history of Fly Fishing Competions was written by Tony Pawson many years ago and second hand copies can be found. Tony went on to become the first ever World Fly Fishing Champion around 1982.
Best Regards
robbie