In your addon file, you would need to create a function that has that script inside of it, and then trigger than function to run on player login.
Something similar to this should work
Lua Code:
local setnameplateframe = CreateFrame("Frame", setnameplateframe)
local function setnameplatesize(self, event, ...)
C_NamePlate.SetNamePlateFriendlySize(60, 60)
end
setnameplateframe:RegisterEvent("PLAYER_LOGIN")
setnameplateframe:SetScript("OnEvent", setnameplatesize)
I have not tested this code and can not guarantee that it works.