Where do i change the dimensions of the fields ?

Is this the form you are working on http://www.jotformeu.com/form/73226690484361?

You can change the dimensions of your fields inside of the Advanced Designer. 

Inside of the Advanced Designer click on the email input field and inside of the Textbox Styles tab change the dimensions of the fields.

I would also suggest you to check this guide:

Form Designer Tutorial: Let's create fantastic forms!