What does this size mean in css {font: 14px/24px Arial, Helvetica, sans-serif}

It’s the shorthand for the different font related properties. It means:

font-size: 14px;
line-height: 24px;

font-family: Arial, Helvetica, sans-serif

See: http://www.impressivewebs.com/css-font-shorthand-property-cheat-sheet/