Forms provided by Adobe Muse aren’t responsive, so to make them responsive you have to resize them for every breakpoint. For each breakpoint, make sure that your form is smaller than your next smallest breakpoint.
For example, Facade has four breakpoints: 1400, 1375, 960, and 650. So, the form on the 1375 pixel breakpoint is 750 pixels wide to make it smaller than the next smallest breakpoint (which is 960 pixels).
Styling Form States
There are a few different form states that you can style to customize your form: Empty, Non Empty, Rollover, Focus, and Error.
Select the form field.
Go to the top left beside Form Field and click on Empty.
Then click on the state that you want to customize.
Style as desired.
Additional help with styling and setting up your form can be found in this provided by Adobe Muse.