Hi NightCabbage, did you save the 3DM file of each of these boxes? That would help if I could see the 3DM files you were using, to double check that they were identical 26x26mm boxes and to see if I could repeat your steps and get the same results. Are you certain you had units set to mm in both cases?

Here's what I tested so far - I created a 26x26mm rectangle, which I've attached as ai_test.3dm.

Then I exported to AI format, using settings like this:



That produced the attached ai_test.ai file.

Then in Expression 3, I used File > Open and opened that AI file, and I got a 26x26mm square in there, you can check the size in Expression 3 by first setting the stroke width of the rectangle to 0 or off (because the width of the stroke is included in the bounding size readout), and then going to its size readout.

To do that, select the square and then push this button here to set no line stroke:



Then under the Transform toolbox, push this button here to get the bounding size controls:



And as you can see there, it shows a size of 26mmx26mm which seems to be what you are looking for...


Are you able to repeat these steps and get the proper results up to this stage in a repeatable manner?

If so then it means that your problem is happening either at the export from Expression or at the import into Inkscape, one of those particular steps is applying some kind of scaling to the coordinates, maybe there is some kind of setting you can adjust to avoid that.

- Michael