The control for default text height is found in the Detail Options. (Go to File > Prepare > Model Properties > and click change next to Detail Options) At the top of the Detail Options dialog you'll see text_height with the default value of CALCULATED. This means that the text height, by default, is calculated as a percentage of the overall size of the model. So, if the model size changes you'll see the text height change accordingly to maintain similar proportion.
If you want to set a static text height, you can replace the value of CALCULATED with a number in model units.
A quick way to determine the current text height so you can get a sense of what number to use is by selecting any annotation and pulling forward the Format tab. The text height field in the Format tab would indicate the current calculated value and you can then use this as a guide to choosing a static text height to use for text_height in the Detail Options dialog.
Hope this helps.
In anticipation of the next logical question, it is not possible to add a column to a family table for the text_height detail option, so unfortunately you can't specify different static text heights for different instances of the family.