btw, you need to open your element in element editor before using it on diagram : add minor change and save or save as, editor refresh element hotspot, it ' wrong by default on DXF converter.

In my opinion you can do this only in circuit diagrams. There is no relationship between the size of two diffrent elements.
If you create a assembly plan this isn't usable. In assembly plan the scaling for each element should be the same. Otherwise the assembly plan is more a good draft then a exact plan. 

Is there planned an improvement for handling assembly plans in the near future?

btw, you could scale your element like this video, element size depend on title block config, increase or decrease dimensions of folio (rows, columns) scale your diagram symbols.

I have a little problem, when I use dxftoqet-converted elements in qelectrotech:

I create a symbol by LibreCad as dxf file. Then I convert this file by DXFtoQET.exe. If I used this converted element in a diagramm, it is displayed very small.(scale factor is about 0.3)
If I compared the values in the elmt-file with the values in the dxf-file, i couldn't find any fault.
May there is a problem how qelectrotech does interprete values of x-, y-coordinate, height, width, length,... in the elmt file. 

I attached the elmt and the dxf file.
(used qelectrotech version: 0.6)

Thanks