As noted, Lightmass will automatically make lightmaps in all the areas enclosed in an importance volume. But what does that mean for your project's memory? You can always check the built data size for a quick estimate (hint: most of this will be your lightmaps, and maybe a chunk for nav-mesh, and a few smaller necessities internal to the level). For example, look in our project's Conent/InfinithBladeIceLands/Maps folder and you'll see frozen cove's built data .ubulk file, about 100 MB. This is typical for what we have in our map. However, if you would like some more detail, click the down arrow (more options) in the lightmass world settings, and under that you'll find a Lightmaps fly-out like so:
And double-clicking on one of those will give you a lot more detail:
Here you can see compression, size (resolution and memory), and of course, change many of these settings. Individual meshes can change their lightmap resolution in the static mesh editor under their mesh settings. In addition, you can view the texel density of these lightmaps in your viewport via View Mode | Optimization Viewmodes | Lightmap Density, as can be seen here:
