There was another issue I ran into that I wanted to share.
I am also using Twitter Bootstrap.
After getting editor.setFontSize() to work properly using your advice, the lines were overlapping vertically.
That was due to a Twitter Bootstrap CSS file setting the line-height property to 18px.
I changed my CSS file to override that like this:
#.ace_editor {
line-height: normal;
}
That fixed it!