/font 9 tahoma is telling mIRC you want to set the font to Tahoma size 9 pixels. Fonts are usually measured (ie. in the font dialog) in points. To set the fontsize using points use a negative number. eg. /font -a -9 tahoma is Tahoma size 9points. Notice that if you set the fontsize using a negative number you will also have to use a switch (such as -a) to prevent mIRC from confusing the -<number> as being a switch.
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