A few points to add:
1. Locus offers resolution-independent scaling for symbols in themes already, if one defines the height in "dp" units, for example:
symbol-height="14dp" scale-icon-size="16,1.25" This sets the height of the symbol in resolution-independent units, and then scales the symbol by a factor of 1.25 for every zoom level above 16.
The dp notation can also be done for widths of lines etc. I have done this in my Voluntary theme.
2. Overall adjustment of the scaling of the map can be used to adjust for individual preferences, using Settings | Map - advanced | Miscellaneous: Increase map resolution. The obvious drawback of this is that it also increases the width of roads and size of text, although the text size can be fine-tuned in Settings | Map - advanced | Vector maps: Text size. However...
3. Even though the map has been upscaled, Locus does not scale the POI overlay symbols for points from its (beta) database and a standard size symbol overlays a bigger one, which is unnecessarily ugly. Request to menion: please sort this!
4. It would be much easier if there was an option in Map-advanced to set a scale for symbols on vector maps. Orux maps does it. It's not the first time someone has asked about this. Request to menion: please add this feature.