JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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There is no direct tool to resize the scale of your background image on the advanced designer, but you can insert a CSS code in it. This guide will tell you how to inject CSS codes into your form: https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes
If you can also give us more information on which background you're referring to and what is your projected size, perhaps we can give you the CSS code to use.
We'll look forward to your update.