Seems like this is the one thing I overlook time and again when working in a new drawing. When I create MTEXT in a blank drawing it never comes out with the right settings on my first attempt. To solve this issue I typed TEXTSIZE on the command line to set the default text height to 0.1.  Note: This setting is stored in the drawing so it would be wise to configure your drawing template with your most used value.
On the ribbon the MTEXT size drop down list defaults to 0.2 and you might assume it cannot be changed.  Not so, read this snippet from an AutoCAD forum post…

Re: Ribbon Combo Box – text height
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01-18-2013 02:28 PM in reply to: evansTSMR
Dean,
Thank you for your reply. The height of the currently active style is set to 0, and the style is defined as non-annotative. I was typing in a new value, pretty much as your screenshot shows, and then clicking outside the drop-down box, whereupon AutoCAD would discard what I typed and substitute one of its own values. It turns out you have to type in the value you want, AND THEN PRESS ‘ENTER” in order for the value to be accepted.

I only found out this critical step by hacking – it doesn’t appear to be documented anywhere. I hope this may help anyone else who encounters this.

Thanks,
Michael Evans

Ready for more automation? Get it right once then Right-click-drag to the tool palette.  All those settings match the next time you start MTEXT from this tool on the tool palette.