It is a common misconception for those not in the sign business to assume all letters and numbers in fonts are all the same size. In fact, the misconception is BECAUSE they are NOT all the same size, rather than that they are. The fact is that the human eye does not do well in perception of round objects next to square objects on a line of objects. If you have a row of round and square shaped objects on a line that are all exactly the same size, the human eye will pick out the round objects as smaller than the square ones. Most people, as I did when I first got into the business, will expect all letters to have the same height when in fact there are very few fonts that do this.
In fact, the text you are reading is not all the same size as can be seen when I blow it up and underline it. Size is important. The bigger the letter or number, the more pronounced the difference needs to be.
Numbers all exactly the same height:
The bigger they get and the further you get away from it, the smaller the "08" will look in comparison to the 4 and 1. To compensate for this the round letters are created larger and float on a center line of the 4 and 1. It is another common misconception that all letters meet a baseline. They do not. Most fonts are designed to use a centerline. This can be seen below
Normal letter heights
By putting boundary boxes like above, the human eye can clearly perceive the differences, much the same as when you physically measure them but without those lines for reference, the very same text in the image below shows how the eye plays tricks ... like Penn and Teller didn't know this .....
Normal Letter Heights again
The bigger the letters are and the further the viewer is from them, the more even they will look.
If you are NOT buying a mounting pattern then make sure you mount the letters using a centerline not a baseline. Letters with round tops and bottoms float equally between letters with flat tops and bottoms. With letters like a "U" that has flat top and round bottom, the tops meet the top of the square letters and the bottom meets the bottom of the round letters.
If you have lowercase letters the same principle applies. The T and Y are level on top and bottom to each other but the body letters are round so they descend lower. Look at the "b" .... see how the flat part sits on the upper line and round part sits below with the rest of the round letters. The tops of the "u" are flat but the "e" and "o" are round so their tops extend slightly above the top of the "u".