Do you mean the physical size or the size in pixels?
The main thing affecting the physical size is naturally the resolution of your monitor, i.e. how tightly packed the pixels are (how many of them there are per inch). There's little you can do about this unless Kindle for PC has a zoom tool (don't know if it does).
The pixel size _should_ not vary between viewers, since I doubt Kindle auto-zooms them (if it does, it would be interesting to know).
The horizontal resolution in kindle is only 600 pixels (maximum). On modern monitors, you may easily have 1980 pixels horizontally, so 600 pixels is rather a small fraction if it.
The main thing affecting the physical size is naturally the resolution of your monitor, i.e. how tightly packed the pixels are (how many of them there are per inch). There's little you can do about this unless Kindle for PC has a zoom tool (don't know if it does).
The pixel size _should_ not vary between viewers, since I doubt Kindle auto-zooms them (if it does, it would be interesting to know).