it's a 28" monitor on 100% res, I've measured the text at 1.38mm.

Less than 1mm text?  The usual font in the Sage 200 forms is Arial 8.25pt = 11px (usually). At 96dpi that's about 3mm. So for it to be less than 1mm, you must have something like 300dpi! or some scaling applied.  I'd be interested to know your settings: monitor resolution, physical monitor size, and scale in Settings,

Of course, I suppose you could be using remote desktop on a phone!

All fields would be of use.

In what fields would you want spell check?