I'm in the conceptual design phase and my plane is supposed to be of 1.7kg. Wing dimensions are 1m*0.3m and AoA that it will fly at will give a pitch down moment of 0.03. The moment arm between CG and tail is 1m. How do i calculate the size of the tail. It's supposed to be a conventional tail.
There's a formula for calculating minimum tail volume coefficient (tail area X distance from the wing) for light aircraft. Not sure if the same value applies to RC but it's a good starting point. This is from David B Thurston's book Design for Flying (Thurston was an engineer on numerous Grumman programs going back to the Hellcat).
(Tail Arm ÷ main wing MAC) X (Tail surface area ÷ wing area) = .55 or greater
Tail Arm is the distance from the C of G to the 30% chord point of the horizontal tail.
.55 minimum is for a symmetrical airfoil section tail typical of light planes like Cessna 150s. I would use a higher number for a flat sheet tail surface, and being RC, so maybe .6 to .8.
While you're at it, Google "tail volume RC aircraft" and you'll find some more information specific to RC that'll be helpful.
This is a great opportunity to compare experimental results with mathematical predictions in a scale model aircraft. Once you have chosen your tail planform, you size it until its torque cancels the forward shift of the wing center of lift during pitch up. If it is too small, the aircraft will continue pitching up and stall (try no Hstab at all and you'll see) Naturally, pitching tendency includes pitch components of wing, tail, fuselage, landing gear, nacelles, etc. The Hstab, as AOA increases, will begin to contribute a nose down force to help keep center of all lifts in the same spot. This work can be done "the hard way" in flight, or in a wind tunnel (safer). Better to err on the side of large. Your 1 meter pitch torque arm should make this model very stable and easy to fly. The next step will be area and throw of your elevator for controlled flight.
Comparing tail design of similar models and existing data is a good place to start. Set your Hstab angle so that when it has 0 degrees AOA (neither up or down force) the wing AOA is optimal (usually for cruise).