This was found on a paid site. It's content so I'm posting it here. If I find more content on these sites that are not posted here I will then post them here for the members of the MMOWNED community so you don't need to waste your money.
/script ScaleInterface(X.X)
X.X is a decimal number for the scale i.e. 1.0 would be 100% or normal 1.2 would be 120% (or 20% bigger in other words) 0.7 would be 70% size (my personal favorite). You have to type it exactly like above capitals and all, and I believe you DO have to put the 0 in there, as in 0.7. It will also take other numbers like 0.85 and such.
You can just type /reload to reload the UI