Tweak the text
Your speech bubble’s font size will vary depending on the size and resolution of your photograph. Doubleclick on the text layer to select the text. As well as increasing the size of the font in points you can click on the Character Panel icon in the Options bar. This opens the Character panel. Here you can adjust the values in the Vertically and Horizontally scale boxes to enlarge the text so that it fits nicely inside the bubble.