As a rule of thumb, the most suitable dimensions for numbers for houses and lettering are 4 inches high. ....But, it also is dependent on the distance from the road you are:
If it's directly close to it, three inches should suffice. If the distance is between 130 and 160 feet you should aim for at least 6 inches. If it's located more than 300 feet the height must be a minimum of 10 inches tall.