Designers need to decide the size and spacing of letters. They measure the size in ‘points’. In digital design a point is 0.3527mm – an apparently odd choice of number that comes from traditional printing measurements in inches.
Point size includes the space needed around a letter. So the total space for a 10-point letter should be 10 times 0.3527mm, which is 3.527mm. The space for a 30-point letter should be 30 times 0.3527mm, which is 10.581mm. A 30-point letter should be three times the size of a 10-point letter. Some typefaces vary in size because they are designed to have different space around them.
Here is Gill Sans doubled three times, from 8 points to 16 points to 32 points to 64 points. If you look at the capital H you can see that the spaces as well as the letters get bigger.