Feature Image Sizing Issues

Hello,

If someone could help me with image problems on my website . I want to change my blog to excerpt posts on the home page but I am struggling with sizing the feature images. I am deciding if I want the excerpt posts to have a thumbnail feature image or a regular sized feature image.

When I tried to do the feature image as a thumbnail, the images came out distorted and would not scale to fit the thumbnail. Is there a way to get my feature images to scale to fit as a thumbnail? Also, if I decide to do regular sized feature images for the excerpt posts, how do I figure out what should be the width of all of the feature images for my blog? All of my images on my blog have a different width and height and I would like them to be more consistent.

Any help would be appreciated.

Thanks!