SIZE sets the size of the font. SIZE can have a value of 1 to 7, or -2 to +4. 3 indicates the "normal" size. The plus and minus values indicate a relative size to the current size.
this code produces this
<FONT SIZE=1>hi there</FONT>
hi there
<FONT SIZE=2>hi there</FONT>
hi there
<FONT SIZE=3>hi there</FONT>

hi there
<FONT SIZE=4>hi there</FONT>
hi there
<FONT SIZE=5>hi there</FONT>
hi there

<FONT SIZE=6>hi there</FONT>
hi there
<FONT SIZE=7>hi there</FONT>

hi there
<FONT SIZE="-2">hi there</FONT>
hi there
<FONT SIZE="-1">hi there</FONT>
hi there
<FONT SIZE="+1">hi there</FONT>
hi there
<FONT SIZE="+2">hi there</FONT>

hi there
<FONT SIZE="+3">hi there</FONT>
hi there
<FONT SIZE="+4">hi there</FONT>
hi there