Hi screenracer. Unfortunately, it is not possible to manually control the positioning of each “object square” in the lightmap.
However, we have another feature that may address your need: Limit Lightmap Count. This feature was made specifically for the use-case of mobile games where you want to limit the number of lightmaps generated. It works by scaling down your objects (in the lightmap) iteratively until they eventually fit into the desired number of lightmaps. Note that this scaling down may affect the number of texels per meter, and consequently, the overall quality. You can read more about it in the manual (search for Limit Lightmap Count).
Here I show how to use it:
Once you have set a lightmap limit, you can continue to work with the “Scale in Lightmap” (MeshRenderer) setting to change the size of objects’ square in the lightmaps relative to each other.
On a more general note, it is no secret that Unity’s packing and unwrapping procedures are in need of some care. For example we need tighter packing and we need to guarantee no texel bleeding between charts. We have several proposed solutions/improvements lined up but we haven’t yet been able to actually set aside time to work on it (except for the Auto Pack Margin feature and the Limit Lightmap Count feature). I cannot promise anything, but I have a hope that we can set aside time for these much-needed improvements in 2021.