It’s inconvenient but it works.  In Internet Explorer I went to Tools / Internet Options, clicked the Accessibility button on the General Tab, and then checked the ‘Ignore font sizes specified on Web pages’ box.  Now I can change the text size using the View / Text Size menu option or using the mouse wheel with the Ctrl key held down.  One drawback is that it changes the text size on the whole browser page, not just in the Write Post text window.