Near the top of your screen (by default unless you moved it), there's a line (possibly more than one) of boxes, one for each window (status, channel, pm/query, and/or DCC)
Point at this box for the channel you want to change (if you want to change them all, I'll get to that), right click it, and select Font...
Change the settings in the dialog box that pops up for that window.
If you want the same settings for all windows and all of the time, then check both check boxes that are in the dialog box.
Click OK
Close mIRC
Re-open mIRC, connect and join the channels...everything should be as you want it.