What steering servo would be considered a drop-in for a Rusty??

Pretty much any that are the right size as the old one

So, standard size, to elaborate on that, not sure of the measurements

Yeah, they don't vary in size in small increments. The next size down will look tiny, and the next size up will be way big.
You should be more concerned about the performance numbers, as those vary alot. I don't know what any numbers are right off the top of my head, but what you probably want is something that has a little bit faster transit time, and alot more torque than the stock.(I'm assuming you have stock in it now)
The torque is the real gain, from my experience. Even stock servos have a decent transit time...with no load on it. If you have the power to turn the wheels under load, then even a slow transit time is an improvement over not having the power to turn it at all.
Maybe somebody has a link to someplace that has done comparisons, cuz I would like to see some real world numbers side by side. But right now I have to go and participate in a scheduled clan war on SOCOM3.[C1S] CrazyOnes ROCK!!