Thanks for the suggestion, Clark, but it's not quite what I am after.
The situation is that I have SE in a single master window with (normally) two edit windows sharing all files and the various tool windows dotted around those. Its size is such that it fits the smaller monitor, which has a relatively low DPI. However, on the big monitor it is tiny and hard to read, so there I've increased the font size to make it readable, and that's necessitated an increase in window size to fit the same amount of info in there. Which, unsurprisingly, has reduced the useful size of the monitor.
So, what I have now is SE24 on the main monitor in large mode, and SE23 set up for the small monitor. The only difference between them practically is the font size. Works OK.
However, sometimes I want more than just two instances going - I might be editing some project and referring to two different projects whilst doing that. I found that the SE23 setup was good in those cases because I could get a couple of instances of SE on the big monitor whilst retaining the normal (to me) proportions of info to windows, etc. OK, I need my glasses more but that's not the end of the world
So that's why I am thinking I don't really need the full 600MB for each instance if only the font size is the differentiator. Sure, I could change the size on the fly as you suggest, but it would rapidly become a tedious chore. Hope that explains things better
The suggestion of macros has got me thinking about external stuff, though - I already use some util that detects a specific window opening and furkles with its size, and it wouldn't be hard to extend that to checking what monitor an app is opening on before Doing Something to it. Hmmmm...