Thanks for your kind reply.
I have checked every option of customizer but I cannot find any helpful setting for either of the problems. I uploaded 225 * 300 resolution images (with a little difference for others) that is why the images are so blurry when enlarged on single pages, and so out of place for the slider. If you can provide me with a CSS short code to reduce the image size for single page and better crop or make some changes to the slider for better image display and user experience, I would be really thankful. Its been a great user experience so far with Mercantile theme.
Thanks.