"k" is the space parameter that controls the size of plots. It is an integer for the following equation: plot_size_bytes = C1 * 2^k(k + C2) where C1 is constant 1 and C2 is constant 2. In practice this means that final size is roughly ((2 * k) + 1) * (2 ** (k - 1)) * 0.762 though that constant is estimated. You can examine the Space Required section of the Chia Proof of Space Construction document for the calculation of how much space is required for a given k.
