Make same height width for each post’s thumb on index

I have Garfunkel theme. I wish to make the thumb of each post in the index page look the same height and width (100% of the width post item) without stretching or modify the featured image on the single post section. How can I do it?

Thanks.

• This topic was modified 6 years, 10 months ago by Dreagnout.

The page I need help with: [log in to see the link]