Click and drag to create an area that contains a series of rooms. Areas whose width is larger than their height will be distributed horizontally, and vice versa.
Click the "-" and "+" buttons to decrease and increase the preferred room size (size is equivalent to width or height depending on the layout).
As an example, say you have the following area in RimWorld:
Click and drag on the screen to recreate it:
Once the size of the area you've drawn matches the one in-game, the preferred room size can be adjusted to find a decent size that is the same for each room:
