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By injecting some custom CSS to your form, you can easily adjust the related areas's width and size.
Below, you will find the necessary CSS rules:
width: 100% !important;
height: 500px !important;
Above, you can change the maximum allowed width by changing the number on the orange area. On the green marked area, the actual width of the form can change (I put "100%" value to extend it automatically when it is allowed; but you can add a "px" value like the others.). Finally, you can edit the height of the form on the blue marked area.
On the following, you can see the final version of your form:
If you need any extra help, please do let us know.
My problem seems coming from preview window. I tried a lot of combination following your advise.
Other question: is it possible to have the buttons on the same line?
It looks better on a physical mobile device:
Unfortunately, the preview tool will not always simulate the mobile view correctly compared to the actual device.
If you need any further assistance on this, please let us know.