What's the optimal size and quality for social sharing preview images?

Hey guys, my store is https://www.clarkssmith.com/, Issue is, I have uploaded social sharing preview image according to the size, and then when I saw the preview it was of pretty kuch degraded quality. A very poor, and then I asked my designer to change the quality, he gave me another image with 300dpi but facing the same issue. I am new here and would be happy if someone can help me in this. Please tell me the exact size and quality I need for better preview like the preview should show my the logo of Company in crisp quality.

Thanks in advance.

You can only use a 1200px x 630px jpg, png (1.91: 1 width-to-height ratio) image here.