You will need to take into consideration two things when deciding house number size. First, where do you plan to mount the house numbers? If you plan to mount them between batten boards you will need to keep in mind any space restaints. Secondly, how far do you need the house numbers to be legible. Standard rule is 4" tall letters can be read at 40 feet and 8" tall letters can be read at 80 feet. Contrast colors and a few other factors will need to be considered as well.
#1 Height: A 4" tall letter will be 4" tall but the width measurement will vary (see below).
#2 Spacing: Keep in mind your spacing. Some like more space between the "No." and numbers.
#3 Width: Below are the width measurements for each style/size.
#4 Overall Width. If you have 56" of space and your overall width for 4" tall letters is 54.2" including desired spacing you know your order will fit your space.