I created a signal with lets say W1 that is fixed at 1 bin in the fft: $$A_1\cos(W_1t)$$

I was asked to find the closest $W_2$ to $W_1$ which the signals $A_1\cos(W_1t)$, $A_2\cos(W_2t)$ are separable.

Now i only know the ratio A1/A2 in db and W1 that I chose.

$$s(t)=A_1\cos(W_1t)+A_2\cos(W_2t)$$

Now my questions are:

1. What is the optimum(minimal) length of the windows?
2. Number of points for the Dft?
3. What (If existing) is the relation between the $\frac{A_1}{A_2}$ ratio to the Dft points?