Comment 13 for bug 1672066

It looks like your issue is related to the difference between "visual bounding box", which includes stroke width, and "geometric bounding box", which does not. dxf output uses the geometric size.

If you click on the Select tool (F1), and select the rectangle, you will see the visual bbox, which is 100mm. But if you go to Fill & Stroke you will see the stroke width is 0.265mm, which is why the geometric bbox is 99.735mm.

When you draw the rectangle using the tool F4, make sure that the size is 100mm. The F4 tool shows the geometric size, while the F1 tool shows the visual size.

Also, you can always get the correct geometric size of an object using the XML editor at Edit->XML editor.