The size of a bulb is determined by the measurement of its diameter. This measurement is expressed in increments of an eighth of an inch.
For an example, an A-19 is 19 eighths inches or 2 3/8 inches in diameter. A 4' T12 fluorescent tube is 12 eighths inches or 1 1/2 inches in diameter. A 4' T8 fluorescent tube is 8 eighths inches or 1 inch in diameter.