1. Well, actually, the marker/font sizes stay the same regardless of zoom (rather than scaling along with zoom), but I know what you mean. This is, indeed, impossible to do without owning the markers yourself (that is, it is something I might do eventually for SPON Map, since other people want what you want, but I don't think you could affect the default markers placed on the vanilla map).
2. The problem with changing the default sizes of existing markers would be that anyone who has made the markers smaller already (by using setMarkerSize in a script) will then get super-tiny markers! Thus, I'd advise against making global changes like that.
If you want to control "vehicle tracking markers", you are better off using a decent tracking script rather than making a lot of effort altering the completely naff vanilla tracking system. Something a bit more advanced, like SPON Sensor (part of PRACS AWACS plane
, but usable separately), might actually suit you better.
The current WIP version of SPON Map (v0.6.0RC6) allows players to choose marker size when they are placed (from 50% to 150% of default size). Yes, I'll get around to releasing it very soon; just keep getting sidetracked!
I'm planning to make marker text to be visible only when you hover over them within SPON Map (yet another thing I'll get around to eventually, but sadly don't expect it in 0.6.0).
Anyway, to affect the default font/marker sizes you'd need to redefine them in class RscMapControl (if you want them changed globally, which would affect all maps, so probably not a great plan) or in class Map within class RscMap (both in ui.pbo::config.bin).