The chord is depth of the wing measured from the leading edge to the trailing edge. With a constant chord biplane that has no sweep in the wings, you just measure from the leading edge of the front wing to the trailing edge of the rear wing. Multiply the chord by .25 to get a centre of gravity of 25%, which is measured from the leading edge of the front wing. So, for example, if both wings have an 8" chord and there is 2" of stagger, (the rear wing is 2" behind the front) then then you have a mean aerodynamic chord of 10" and your balance point is 2.5" from the leading edge of the front wing. (10+2) X .25 = 2.5 The 25% is only a safe starting point. It is likely to be somewhat nose heavy, but you can move it back a bit at a time in subsequent flights. Better nose heavy than tail heavy because a nose heavy plane flies poorly, but a tail heavy plane flies once.

A .46 2 stroke or a .70 4 stroke would work nicely.