How to change size of field

If it's a textbox field, you can change its size in the field properties. For example, in the email field:

Properties > Options > Width:

The height can be adjusted with a custom CSS codes.

https://www.jotform.com/help/75-Customize-Your-Form-Using-Custom-CSS-Codes

https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes