So I buy a plane. Then I need a radio and servos.
So if I buy, say a Futaba 6 channel kit that comes with servos. Will the plane instuctions tell me if I need, micro, mini, or standard servos.

Or is there some other "rule of thumb"?

Almost ANY plane you buy will recommend servo size. Be it standard, Mini, Micro, Heavy Duty, or a combination of the above.

What type of plane are you looking for/at? Do a search here on that craft, chances are you'll find some input or can pose a question.

If your getting a trainer (.40 - .60) you'll be okay with standard servos. Park flyers require (most of the time) minis or micros, and larger aircraft/stunt birds require heavy duty servos on the elevator/ailerons.

BUT, the plane specifications/requirements should give you a good indication of what is required as a minimum.

Backwing,

You'll hear it a hundred times here and as a newbie, I can attest to the fact that it's good advise - find a local club call them up they will let you know who does instruction and find out from them what kinds of radios they can "buddy box" with for instruction purposes.