questo è quanto descritto per l'utilizzo del tag <font> nei livedocs di flash 2004

codice:
Font tag (<font>)

The <font> tag specifies a font or list of fonts to display the text.

The font tag supports the following attributes:

    * color Only hexadecimal color (#FFFFFF) values are supported. For example, the following HTML code creates red text:

myText_txt.htmlText = "<font color='#FF0000'>This is red text</font>";

    * face Specifies the name of the font to use. As shown in the following example, you can specify a list of comma-delimited font names, in which case Flash Player selects the first available font:

myText_txt.htmlText = "<font face='Times, Times New Roman'>Displays as 
either Times or Times New Roman...</font>";

      If the specified font is not installed on the user's computer system or isn't embedded in the SWF file, Flash Player selects a substitute font.

      For more information on embedding fonts in Flash applications, see TextField.embedFonts in Flash ActionScript Language Reference and "Setting dynamic and input text options" in Using Flash.
    * size Specifies the size of the font, in pixels, as shown in the following example:

myText_txt.htmlText = "<font size='24' color='#0000FF'>This is blue, 24-
point text</font>";

      You can also use relative point sizes instead of a pixel size, such as +2 or -4.
la seconda domanda trova risposta a quanto pare nei riferimenti relativi a punti, utilizzando davanti al numero un + o un -

per la prima domanda il dubbio è che tu non abbia usato un TextFormat che ha "imposto" un determinato font, in tal caso devi eliminare quella riga ed assegnare sempre il font via html