Let's assume the 30 million points in the LiDAR cloud has a point density equal 10 points per square meter (m²), and therefore, the area of interest is of size 3 million m².
Here are some references to help you finding the point density: 1, 2, 3.
Now, for the sake of simplification let's assume the area of interest has a squared shape. This is because the
lastile tool uses a square tiling approach (the reasons for this are provided in the link above).
Then, if you want the original las file to be splitted in 6 smaller ones, each tile will have area equal to 3 million m² / 6 = 500 thousand m².
That means the dimensions of each tile are approximate 707 m x 707 m (square root of 500 k m² = 707.1067812 m).
Finally, the command-line parametrization would be:
lastile -i input_file.las -o output_file.laa -tile_size 707