file sizes depend on the data being written into them, some formats are easier than others to predict but in general it is possible to know a ballpark number of the file size depending on the amount of polygons the mesh being exported has, this does not have anything to do with the world-coordinates-location of their vertices if that makes sense.
You could have a mesh where all vertices are overlapping each other at the origin (0,0,0) and it would still take up just as much as with all vertices in their positions.
Scaling up/down should not cause any detail to be lost, if it does, something else has gone terribly wrong. A scaling operation is only a matrix * vector multiplication, as long as the matrix isn't wonky it should come out ok.
A separate issue would be that a human head model with a polycount of about 2 million printed at 1/6th scale size would look super detailed... but if you make it 1/2th or 1/1th scale you might find it needs "more" detail, in the 50+ million range (to say something) but then again, the scaling operation shouldn't affect vertex positions (there should be enough range for floating point accuracy to not mess up)
Since you mentioned Chitubox, I must say I use it as well, and OBJs load fine too, I personally prefer this format as it works well on all 3D softwares, but files are going to be certainly larger than binary STLs. 😉