Hi,
It would be nice you can set horizontal and vertical relative sizing and positioning to a group of objects or (and this would be really nice) to all your objects at once.

Have you tried making a permanent group of the objects -- then applying the positioning/sizing to the group?

If you want all objects in a project to be set with relative positioning/sizing, see the main menu Options > Project Options > More > Default Positioning.

I've made permanent groups of my objects and indeed it is possible to set relative sizing and
positioning when you check "Re-render" at the object properties.

But when you do this for the group in a drop down menu it becomes a mess when previewing in the browser. objects don't behave as they should; disapear,move slightly etc.
I think practically spoken it is impossible to
make a "stretched" site with drop down menu's.

Do you agree with that?
Or do you know a method that works?

Yes I would agree with that. But that is not to say it can't be done!

I find relative positioning and sizing to be a huge hassle, so avoid it wherever possible. I use horizontal centering instead -- Page Editor > Special Effects tab.

I think it can't be done. I've tested it and isolated the drop down menu from the tutorial on
a single workpage and then added relative positioning and sizing to each part/group Result: It doesn't work.