You don’t have to do a thing and your forms will automatically adjust and look awesome across all devices including desktops, tablets, and mobile phones.

Fixed Width Form - You can always specify a fixed width to entire form from Styling > Form Width which would turn off the responsive behavior.

Field Width - Several fields have their own width property which by default is set to 0 for responsive behavior. Changing these to a fixed width will force the field to render at specified width and ignore the responsive behavior.