I had a Kent and masters for a few months. I sold it as my knees kept coming off the knee rolls in normal stirrup length. I'm tall and long from knee to hip.
Not odd at all, saddles vary massively, especially in flap design. Long femurs do indeed need a more forward flap, a high place thigh rather than knee block can also help. A larger seat size is not the first choice for this as it can leave you "swimming" or sitting in the wrong place, flap design is more important.
Can't help on seat length I'm afraid. And it's all very well saying go up a seat size to avoid being tipped forwards, but it seems very common that people are experiencing this being tipped forwards (and usually that is due to the saddle being out of balance and poorly fitted, but it seems that can't be the case with the numbers involved) and not all horses can take a larger seat size!
I have a thorowgood GP saddle and I'm 5'6", size 10-12 with rather long legs. My last saddle was a 17.5, but that came with my last horse so I didn't think to change it; when the saddler fitted the thorowgood to my new horse she said that I only needed a 17. And it's actually far more comfortable - I think that I can sit slightly deeper and stiller. I never thought a 17 inch would fit me though!
I'm 5'9 and ride fairly short all the time, my horse is 15.1 and short backed (5'9 rug) - my K&M is a 17" but it is also a jumping saddle so more cut forward in the flap. I am certainly not at my slimmest at the moment (size 10/12) and i am fine in my saddle, find it fairly comfortable however i do feel that i rode a lot better in my old saddle - but i think thats related to the fact that this saddle is a jump and old saddle was a GP.
Like other people who have commented, I found my Kent & Masters cob GP saddle tipped me forward and wasn't actually that comfy. I switched to an Albion K2 GP and the difference was incredible...so comfy! I had a 17.5" K&M and it came up true to size. The one bonus with them was the adjustable gullet which was very useful - acquired a "project" that had been out of work and seemingly ate his own bodyweight in grass whilst off so ended up changing the gullet bar down from an xxw to mw over time as he got fit again!
i have the 17inch gp k&m saddle... horse has high withers and a hollow behind the shoulder he is 17.2hh but can only just about take a 17inch ideally should be in a 16.5inch(hes 6'6 rug size so you get an idea of how short he is ) im 5'4ft size 10-12 and have relatively long legs... i fit in the saddle fine... they come up bigger than a normal 17inch but it does tip me forward dispite it being correctly fitted and checked every 3 months :/