If you want a larger render, you have to first set up your render dimensions.  At the top, click on the Render selection, then click on Render Settings.  Set up the size of your render, then make sure to select "Render to: New Window". 
A thing to remember, the larger the image, the longer the render is going to take ;) 

Sitemail | Freestuff | Craftythings | Youtube|

Knowledge is knowing a tomato is a fruit. Wisdom is not putting it into a fruit salad.