As far as I know, there are several parameters that you must take care of to get the text size where you want it.
1.
If the drawing was created in desktop AutoCAD and you're working with it using web AutoCAD, I have no idea if this will help you or not. The web version may have difficulties reading the styles setup in the desktop version.
2.
Make sure you are aware of the template (either metric or US customary) that you started the drawing with, or that the drawing is based on. You will fight the drawing if you are trying to use inches when the drawing started in metric, for example, and vice versa.
3.
Before starting the text tool, open Object Properties tab and set the Text Style. I keep it simple and use Simplex. The trailing number is the scale factor. So, Simplex_0.25 will be small, and Simplex_12 will be very, very large. Those are example styles from an imperial-template-based drawing. Metric styles are like Arial_3 or Arial_25. The same general rule of thumb applies with the trailing scale factor determining the initial text size.
4.
After you have clicked in the drawing or layout space with the text tool, you will see a word processing looking interface which has a size field. You can use this to set the text size also. I'm not sure how far you can push a size determined by style versus the size determined by the text size field. Experiment and you'll get it.
I hope this helps.