Pull out your calculator, even the windows one will do, choose an appropriate diameter and radius for your planet, the formula for the volume of a sphere is: V = 4/3 * pi * r ^ 3. The volume of a hemisphere is half that. Calculate your volume. Each block in starmade is one metre cubed, so your volume is also your number of blocks. Make a ship with that many blocks, and decide if you like your framerate.