Actually, I have found the lightmap in \img\large\std folder. I tried to manually resize it but this was not the solution 
What if I set lightmap resolution to something about 50, would it generate smaller lightmap?
Or it won’t work on older devices anyway?
What might be the limit for older devices (I’m planning to use an HDRI map so need to know if f.ex. 3500 px will do or it has to be no more than 2048)