I have in mind something like that for a 30cc gasser.... I made some numbers a few time ago, but I can't find them.

Well, I'm not the best person to speak, but just to give an idea... Rotor loading and rotor size vs engine size seem like the main things to watch out for, since they are quite different from those on a plane. While a plane can usually fly with low/excess power/wing area, autogyros are more critical to that. In my experience, a very low rotor loading is advisable, as excess weight soon "kills" the gyro capabilities and behaves similarly to a plane; the machine has to fly fast to have enough lift and you virtually lose all the low speed advantatges they have. You can't just cure a heavy craft by adding rotorspan, as it also adds drag and other forces which your frame/components might not withstand.
I think you can get a basic idea of dimensions by finding cessna-type planes which use your same engine, but keep in mind that your craft must be even lighter, so you'll likely keep the wing (rotor area) and shorten somewhat the other dimensions.

Using heli blades isn't a good idea, IMHO. You need lift, lots of it. A symmetrical blade will give you very poor performance; Jim is very right about that.

By the way, what do you mean with "main roto with a direct drive gear" and "uncontrollable tail"?
Anyways, I wish you luck with your project!