Try the line-height property.
For example, 12px font-size and 4px distant from the bottom and upper lines:
line-height: 20px; /* 4px +12px + 4px */
Or with em units
line-height: 1.7em; /* 1em = 12px in this case. 20/12 == 1.666666 */
You can also use a unit-less value, which is the number of lines: line-height: 2; is double spaced, line-height: 1.5; is one and a half, etc.