The 'size' parameter picks how big a mapblock to use and can be either a single number, e.g. size: 2 means to use 20x20 blocks, or a list of [width, length, height]. The 'freqs' parameter is a list of weights for the 'blocks:' part of a command; it allows you to set the relative frequency for each of those blocks to be chosen for a command. blocks: [2, 3] with freqs: [1, 2] means pick block number 2 one out of three times and pick block 3 two out of three times.