I got the idea today of building a Malmsteen-esque Strat (Won't be happening soon however) to save myself the $7500 and I remember him saying he only has the neck and bridge pickups wired and the middle one is just for looks. Since that's the case, could I put a 3-way blade selector in the place of the normal 5-way and wire the two pickups to it or are the 3-way selectors bigger than the 5 way ones?
Yes, you can. No, they're the same size.
Earlier Strats all had 3 way. With care you could find mid points where two pups were alive.
You could always just add a push button to activate your neck pickup or wire your five way with just the two pickups and then a couple "dead spots".
nah, i hate middle pickups and never use them no matter what so it would be mounted flush with the pickguard and out of the way; i dont use pickups them in tandem either, its neck or bridge for me
They are the same size. You'll have to use both poles of the switch. In my UWD thread (sig), there's a diagram of how to do it, though the tone/volume wiring is different, but works the same.