Mark - from my experience, the average fish on the flats are between 10 and 20 kilos, but there are certainly plenty of fish upwards of 20 and into the 30's to be had and seen...
On my trip, the two biggest fish went 118cm and 122cm respectively.... but we saw plenty of fish that dwarfed even these.... watching a 45 kilo plus GT churning up stand and marl in less than 2 feet of water while hoovering bonefish, while standing with a 9 weight fly rod in your hand as this behemoth swims straight at you and past your feet less than 2 metres away is a difficult thing to describe, and the feeling of helplessness with that 9 weight in hand is quite humbling...
There are plenty of 50 kilo plus fish in Seychelles, and some that will push well above that mark...
The fact that its flyfishing only means that less of these fish are targetable though, as I think the bigger fish don't come right onto those shallow flats as often, and will spend more of their time in the slightly deeper, and more violent areas such as the reef passes and outer edges of the atoll...
The biggest I personally saw on our trip was a fish that I would battle to call size wise, but I was on hand when the 122cm fish was caught ( not by me unfortuntely ) and the fish I saw dwarfed that 122cm specimen... it chased a fellow anglers fly head on in the surf zone and he actually pulled his fly away from it at the last moment in a form of sheer panic.... the guide went ballistic of course, but Danie, the angler, could not even lift the rod or stutter a word of excuse for about 30 seconds as he just stood there shaking with his jaw going up and down like a fish out of water..... after nearly a minute all he could say was " But, but, but, it was as wide as you!" to the guide.... it was a big black GT that was seriously at least 55 kilos, in water less than 3ft deep.... dorsal and eyes out the water as it chased the fly.... insane stuff...
Add to this the HUGE doggies that live there, Napoleon Wrasse on the Flats, and more bonefish than anywhere else on the planet, along with Triggers and Bumphead Parrots on the flats.... its a mad place.... with a mad price tag...
Check out this gallery of the 2010/2011 season on the fly...
http://www.flycastaway.com/image-gallery/2010-2011/farquhar-2010-2011/