how do i get a thumbnail image to appear properly in the main blog image slider?

I installed a WH Theme recently. After I upload a thumbnail image into a particular post, I select a feature image. My new theme has a main blog slider widget. The problem: The image that appears in the slider does not fit properly. It is too small. How do I get the image to fit properly in the slider. Note: I did upload 940 x 400 size images only.