Forgive me for asking because I tend to use one racquet ever since and I really don't know the head size but since I have many of the same model I haven't changed myself to the newer frames. I have tried other racquets of course. My brother uses a Wilson which I believe is way easier to play with. My question is for a guy like Fed who is switching from 90 in-sq to 98 in-sq, will a difference of 8 in-sq make a lot of difference and if so what are the biggest advantages and disadvantages.
larger head size = larger sweetspot, so less mis-hits presumably.
larger head size = larger surface area to impart more spin to the ball. This should allow him to hit harder on the ball and as long as he puts enough spin, it'll keep the ball in. Not sure how this might imapct him since he already puts a lot of spin on the ball, but I can see this giving him more margin on his ground strokes.
larger head size with the same tension he's used to = more spring action from the strings when the ball comes off it, so greater power, at the cost of accuracy.
larger head size = more mass above the midpoint than a comparably weighted racuqet with smaller headsize, making it less maneuverable at the net. Shouldn't be too much of an issue considering the game isn't net-based anymore and the minor difference is more than negated by Fed's skills which ultimately determine how good your volleys are.
those are the advantages of larger head size. On his serve.. that'll depend on the weight of the racquet, and how his service motion might be affected by it. I personally don't see how a larger head size will impact his serve, but the above statements defintiely would have an impact on his serve.
I have a feeling he'll make the racquet just as heavy as his PS 90, but if he chooses to make it lighter:
lighter racquet = potential to swing faster. Momentum is a direct relation to mass and velocity, so wether the decrease in mass justifies in the increase in swing speed to net a higher momentum is to be seen.
lighter racquet = more vibration. This shouldn't have too much of an impact since Fed doesn't have a history of wrist/arm/shoulder issues.
lighter racquet = less resistance to twisting, again for an athlete as strong as Fed, I think this should have minimal impact but the ball probably won't feel as solidly struck to him until he gets used to it.