This is sadly becoming such a difficult decision for so many and I wish it didn't prove to be so challenging. But the fact of the matter is there is very very little scientific study on the subject that "proves" much for either side of the argument. Until there is, it will be controversial. In the meantime, everyone elects what they read and want to follow - not a bad thing in lieu of any credible information.
Many against early spay quote Chris Zink, who has compelling arguments, but likes to cite a lot "research" here and there that is highly misleading and over-stated. Vets on the other side however also like to say things like "proven to have no affect". This is no more compelling or substantiated.
The only broad and credible study we have states that spayed animals will grow 1/4" longer in the long bones (legs)and therefore be "slightly taller". There is no evidence it is any more significant than this. Many agree that the lankier look in many large dogs is a telltale sign of spay. Now if you are highly involved in performance events (agility, flyball etc.) then this can be very meaningful. If not, it is highly unlikely this will matter. This long bone growth is why Zink recommends waiting until 13-14 months - when the long bone is generally finished growing (although the growth plates often have not closed). Much more goes into structural health and performance - the structure of the dog and how well the dog meets its breed standard is most critical here. If you have a dog with poor angulation in the rear, it won't matter when you spay - he will still be at risk for physical injury.
Hormones certainly play a clear role in the degree of masculinity and femininity in the dog - this is mostly expression/face, breadth etc. Spay does not affect other skeletal growth - size of head - or coat growth.
If your concern is health however, there is growing intuitive belief that the immune system is best developed and regulated with hormones intact - although no studies have been performed in canines. We know that in humans, a child that loses her adrenal glands early receives significant hormone treatments to carefully manage growth and normal development. It stands to reason that the canine is better off not having its hormones removed prior to immune system maturity - in large breeds this occurs at about 9-10 months of age.
I am a big proponent of spay/neuter and think it is important societally. I have had dogs spayed at 8 weeks and 6 months and everything in between. I have never waited longer. If I get increasingly larger dogs (who take longer to mature including the immune system), I will likely feel compelled to wait until after 10 months to do so in the future strictly for immune health. I am not concerned about physical development since I am not a breeder and do not show. I focus instead on acquiring a dog with sound structure that meets the breed standard (often not possible in rescue) and if I don't and am aware of structural weakness - I will be more careful and watch this dog for injury.
I agree with most posts here already - the performance considerations, the importance of spay/neuter, and most importantly, the tremendous difference in recovery time and risk of complications the older the animal is. At 8 weeks old, they are playing 2 hours later. At 4 months old, they are playing the next day. And at over a year, the risk of complication and degree of pain is much much higher.
In the end, I think you should do what your heart and inclination is - otherwise you can only regret. If you are comfortable with waiting a bit longer, do not have behavioral issues or raging hormones, and are a responsible owner, then do what makes you most comfortable.
