You can select a font size for the text tool before you add text by moving the slider in the text tool section of the tool picker to the desired font size.

To change the size of text that is already on the page, you must first select the text. Then tap the weight / thickness icon () at the top of the screen, and select a new font size for the text you selected.

For a video tutorial about changing the text font size, see Squid - Tools Overview.