All typefaces are not created equally. Some are fat and wide; some are thin and narrow. So words set in different typefaces can take up a very different amount of space on the page.
The height of each character is known as its 'x-height' (quite simply because it's based on the letter 'x'). When pairing typefaces – such as when using a different face to denote an area of attention – it's generally wise to use those that share a similar x-height. The width of each character is known as the 'set width', which spans the body of the letter plus a space that acts as a buffer with other letter.
The most common method used to measure type is the point system, which dates back to the 18th century. One point is 1/72 inch. 12 points make one pica, a unit used to measure column widths. Type sizes can also be measured in inches, millimetres, or pixels.