What is the standard paper size and what is the code to make my membership form that size?

US Letter size paper is a standard in the US - https://en.wikipedia.org/wiki/Letter_(paper_size) 

A4 paper is a standard for all other nations - https://en.wikipedia.org/wiki/Paper_size 

http://www.a4papersize.org/a4-paper-size-in-pixels.php 

Inside of this guide you can find how to change the width of your form: 

The Importance of Form Widths

The height of your form will depend of number of fields. Most likely you will have to position the field inside of your form and to adjust the spacing between fields. 

Form Field Positioning

How Can I Change Spacing Between Questions?

How to Reduce Vertical Spacing Between Fields

Hope this will help.