1. <font> tag has been deprecated. This means that the tag is no longer a part of the HTML specification. While browsers will still support it, most likely for a long time, it's a good idea to switch to the alternative.

 

2.

TECHNICALLY <font> tag is supposed to be a scale from 1 to 7, but the definition of that 1 to 7 is NOT defined in the standard - though thankfully most browsers today use the same approximate values.

Those values being:

1 = 62.5% = 10px
2 = 83.3% = 13px
3 = 100% = 16px
4 = 112.5% = 18px
5 = 150% = 24px
6 = 200% = 32px
7 = 300% = 48px

Mind you that's assuming you are in small fonts/96 dpi and your browsers default font size is 16px, as with large fonts in IE and Opera the px values would be 25% larger.