How can I make the text for a submission label larger? The default is very small

The current font size for your form is very small:

Here is a guide that will help you to increase it: How-to-change-font-size-on-forms

If this still persists, you should also update it on the email body, to edit it you only need to click on the email button: 

Once on the wizard you will be able to change it by using the tools of the text editor: 

Hope this helps.