[zork]

I checked FreebGrid. He is doing

local setHeaderScale = function(scale)
    for k, v in pairs(ns._Headers) do
        if v then
            v:SetScale(scale)
        end
    end
end
 
function ns:scaleRaid()
    if not ns.db.scaleYes then
        setHeaderScale(ns.db.scale)
        return
    end
 
    local numRaiders = GetNumGroupMembers()
 
    if numRaiders > 25 then
        setHeaderScale(ns.db.scale40)
    elseif numRaiders > 10 then
        setHeaderScale(ns.db.scale25)
    else
        setHeaderScale(ns.db.scale)
    end
end
 
local updateRaid = CreateFrame"Frame"
updateRaid:RegisterEvent("GROUP_ROSTER_UPDATE")
updateRaid:RegisterEvent("PLAYER_ENTERING_WORLD")
updateRaid:SetScript("OnEvent", function(self)
    if(InCombatLockdown()) then
        self:RegisterEvent('PLAYER_REGEN_ENABLED')
    else
        self:UnregisterEvent('PLAYER_REGEN_ENABLED')
 
        ns:scaleRaid()
    end
end)

So basically waiting for out of combat to change the scale of a raid.