If that’s all you want to do, you could:
try
/run SetCVar("uiScale", 0.5)
I’m not sure if it’s per character or sticky at that scale.
Otherwise, paste the following into the website addon.bool.no to create/download a small addon that will run the script automatically each time a character logs in.
local f = CreateFrame("Frame")
f:SetScript("OnEvent", function()
UIParent:SetScale(0.5)
end)
f:RegisterEvent("PLAYER_LOGIN")