Set Featured Image size

Hello All.

I am trying to get a new theme set up, it uses features images in select posts to show on a slider on the homepage.

My problem is when I set a featured image it also appears as a thumbnail size image in my post. Using the set as featured image ignores the image size and alignment settings.

I have been able to change the image size in the css, however doing this lost a lot of image quality, which to me implies the images was just shrunk and then enlarged with lower quality.

So for the questions, any of these would solve my problem

1) How can i set a featured image but not have it show in the post.
2) How can i get wordpress to not ignore the size & alignment of the featured image

If you are able to answer this question please be as specific as possible, I have some programming experience but virtually none with php. I also understand that the coding you dont know the coding in my theme.

Thanks in advance for any help you are able to give me.
Josh