While volunteering at a Habitat for Humanity project, I found out that the inspector said the house numbers that had just been nailed on were too small and had to be changed. They looked big to me (I could read the numbers from over 50 feet away) and I wondered why a store would sell house numbers that were too small.
It took some searching, but I found the regulation in the Montgomery County Code. I thought it would be in the building code chapter, but in chapter 22 Fire Safety Code section 97 it says “Numbers must be at least five (5) inches high for single-family detached and attached residences.”
So consult a professional and double check the codes in your area for yourself if possible!