I have an object with multiple MeasureIt annotations. When I render MeasureIt and look at it in the compositor the font size is way bigger. Override is turned off, all font sizes are the same. Render format is at 100%. Image size 1200 x 1080. When I do use Override, the same problem happens. Does this have anything to do with the camera settings? I am absolutely stumped.
Interesting… i just learnt that you can render the Measure It–measures… 
I guess because the measures are drawn on the viewport and depends on the size of the window you have to open a window in the size of the wanted image (which is a bit of impossible if you want to render a full screen or more sized image) to see the “actual size”.
You screenshot already shows “more than the camera can see” … so there is some missalignment.
You maybe also ask in this thread (the orignal repo seemd to have been merged):
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IDK if there is some measure tool which do use real 3D objects and so using the WYSIWYG approach… said this… maybe (?):