Just solved it (not very clean). I'm running Arch and I just hacked the /usr/bin/sweethome3d script.
I've just added the -Dcom.eteks.sweethome3d.resolutionScale=2 in the options of the launch command and it worked like a charm.
I guess I'll have to redo the same after the next update ) So I will document it right now.
Hope this helps
Jean-Pierre