Haven't tested it, but I believe this should do the trick. I don't know if forbidden frames would block access to fontstrings, let me know if it does and I can tweak it to avoid them.
Lua Code:
hooksecurefunc(NamePlateDriverFrame,"ApplyFrameOptions",function(_,nameplatebase)
local castbartext=nameplatebase.UnitFrame.castBar.Text;
castbartext:SetHeight(0);-- Remove height limit so vertical truncation doesn't happen
castbartext:SetTextHeight(8);-- Set to whatever size you want
end);
(Answer to same request via PM)