Photoshop :: Screen Shots With Consistent Size

I want to use screenshots of websites in a web based portfolio.  The images should be about 200px * 300px, or so.   I'm having a hard time producing consistent results when cropping.  I can do a screen shot of the entire screen for all of the sites and change the image size. The issue with this is that the shots are so small that the detail of screenshot is lost. This creates a very low quality image.
 
I tried to solve this by cropping the screen shot to a only feature a portion of it which would produce a more "artsy" looking image that would show the details more clearly.   It was difficult to produce images that were sized consistently.  There was no way to know what the exact size of the cropped image was.
 
Can I define a size for a mask and then apply it, or reverse it, to have a clearly defined size?