Well, here's my take on it. Unless someone has measured the size from an actual Rogue One screen-used prop we're left with using the available photo ref to guesstimate. This image from page 169 of the Rogue One Ultimate Visual Guide provides a fairly face-on view of this greeblie:
View attachment 799981
My approach to these things is to take the average adult male pupilary distance (64 mm -
https://en.wikipedia.org/wiki/Pupillary_distance) and use that as a scale bar to measure the sizes of other things in an image. I suppose one could also in this case use the known scale of features on a rebel pilot helmet. Assuming this actor has the average pupilary distance the original version of this image from my camera sets the scale:
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We then measure the dimensions of the greeblie and convert:
View attachment 799983
The millimeter dimensions labeled here are actually rounded up so a window size matched to that ought to give a bit of margin to fit the greeblie inside.
It'd be great if someone has confirming or more reliable data from the actual prop. I'm open to other interpretations, but this is my approach...
Dan