There is a setting for default font size but i think it would be better to have a default font to width. This way anyone could enter text and it would always fit to the canvas but the font size could then be adjusted.
At present you can set the font to width in the main settings but there are a few bugs with this. Namely that you can't change the font size using the actual setting but can still drag the corner. This then means that the thumbnail for the order looks correct but the version that loads into the order viewer will resize all the text and is not usable.