Traditionally, letter type size is designated in points and is set to specific pica widths and depth (Multiple standards exist)

Point Size:

• 72 points (6 picas) ≈ 1 inch (72 points = .996264 inches) - Normally rounded to 72 points = 1 inch
• 1 point = .013837 inch
• 1 pica = 12 points (6 pica/inch)
• A font will normally also include spacing. In example, a 12 point will be 18 points total height ("12/18")
• X-Height is the height of the lower case "x"

In Microsoft Word, paragraph spacing, 96 pixels = 1 inch