The head size designation on frames is really more of a number than an actual measurement. If you hold a 90 up to a 95, there is very little difference. If you hold a 95 up to a 98, there is very little difference. And, if you hold a 95 up to a 100, you won't see much difference.
The real focus then shouldn't be on head size, rather what you play best with. Generally, there is the pro-90 square inch camp and the anti-90 square inch camp. The pro-90 camp generally feels they can only play with a 90 and the anti-90 camp generally think the pro-90 camp are all wannabes.
Historically speaking, the first OS racquet was the Prince. Before it was the Prince Classic (the introduction of the Pro gave it a name), it was just the Prince. It was 110 sq inches and everyone laughed at it.
Then folks started buying it. Prince had a patent on 110 sq inch frames so the other guys started making smaller head sizes. At first, they made wood mid sizes which ran 80 square inches.
The first iteration of non-standard tennis racquets were 80 square inches. The 85's came into play around '84. Since then, there has been a gradual progression to a larger "standard" size. Today, the 95 is becoming rarer and rarer and the market is dominated by 100 sq inch frames. The reason for this is two fold. First, manufacturers have learned to tame the power on a frame and build a modern frame which bears no resemblance in weight and balance to its wood ancestor. They are easier to swing, easier to play with, and can be tailored for virtually any level player. Second, the younger set of players typically play with a bigger racquet as kids. They may change a little, but not a lot as they get older and get used to larger head sizes.
Personally, I've fought the urge to play a 90. Most of it is nostalgia on my part (I think). I will say this, when last I played a 90, the Dunlop 100, it was a blast to play with, but truth be told, you had to be playing a lot to play well with it. I developed Achilles tendinitis and had to get off court for a couple of months. When I came back, I found I needed more room for error and I broke out the C10s.
For the last couple of years, I've been playing with a 100. Upside is it's easier to hit with, down side for me is just the serve.