I will try your last idea later. The implementation has to be secure. Otherwise it is of no use since raid headers can be spawned in combat.
Maybe I can do this:
Lua Code:
"oUF-initialConfigFunction", ([[
self:SetWidth(%d)
self:SetHeight(%d)
self:GetParent():SetScale(%f)
]]):format(110,26,0.5)
*edit*
Tested it. The above is the solution!
It works if you set the header scale instead of the unit button scale inside the initialConfiguration.