Thank you for the posting--all I collect is Newfoundland so nice to see.
Posting an image would be helpful, so do that if you can.
In general, the best publication to use is Walsh's Newfoundland Specialized Stamp Catalog, as it has more details than Unitrade. This stamp came in yellow green and blue green and the EKU is June 18, 1898--(though Boggs listed only green, many people debate the various shades. I personally don't get too hung up as time has a huge impact on color determination) .
Both stamps were 12 x 12 perf and with over 14.25 million stamps printed.
As far as the measurement, I have not seen anything in the literature on this variation but its not clear to me are you talking about the image size or the stamp size? I would think the ideas above about shrinkage (especially if they are used stamps), or time would explain a wide variation, even 0.5mm. I'll check a few of mine--i have about 15 used and 5 mint versions.
here is an image of my two mint stamps, which show the color differences : again yellow green and blue green...
