Are you designing a form in FormsCentral to distribute a web link or PDF?
For web forms you can change the width of the page under "File" - "Form Setup" (from the Design tab). It is set to 700 pixels by default. The height is controlled by where you add page breaks in the form.
Note: you can also resize the width by click/drag on the right margin.
For PDF, in the "Form Setup" options (button on the top toolbar in the Design tab) you can choose a Landscape layout there that would be used if you save the form as PDF.
I hope that is helpful, let me know if you need more details.