How can I resize text added to a jpeg?

As X423424X has indicated, if you are using Preview to do the annotation, once the doc is saved and closed you can not alter the added text except by editing the image - or starting over.

The trick with Preview is to make your adjustments before saving and closing the doc. As long as the text box is selectable, i.e. you can click it and have the box surrounding the text appear, you can alter the text - its size, color, and font. You don't need to select the text itself - just the text box. Then use the Show Fonts item in the Tools menu to access the font controls and make your changes.

Once the text is added to a picture file (say, using annotate with Preview) and saved it becomes part of the picture, i.e., just more pixels. So the only thing you can do at that point is edit the picture. One good such tool for this kind of thing is GraphicConverter.

If yo use Annotate in Preview you can change the font and fint size of the text you want to annotate with using Preview's Tools Menu, Show Fonts. For example below is a piece of your post I too a picture of and overlayed a 72 point chunk of text.

Any semi-decent picture editing tool that allows you to add text to the picture is going to have some kind of GUI to allow font control.

Note, I got curious how you change the blue annotation text color. That's controlled by the color menu annotation tool.