I don't see any problem here.
Here is my test file:
A0test.val (2.2 KB)
This is rect 800x1100 mm. A0 format of paper sheet.
And here is result in Inkscape. I have been using version 0.92 and the latest Valentina in branch 0.6.0.
Result in pixels:
Little bit of math.
800 mm / 25.4 * 96 (dpi) = 3023,622047244 px.
Inkscape result 3024.99 px.
1100 mm / 25.4 * 96 (dpi) = 4157,480314961 px.
Inkscape result 4157.99 px.
As you can see difference is 2 pixels for width and less than pixel for height.
I believe you get wrong result because the tip of curve is not the best choice measure to.
Also i exported the file to pdf and opened in Inkscape. Here is result in this case:
This is actually good news because we use the same code for exporting in all cases. Scaling is correct. Looks more and more that you don't left enough space for printer to print your layout. Or our printing settings are somewhere wrong.