Hi Bennity,
I hope you are well today and thank you for your question.
It is different for various thumbnails as listed below.
Normal thumbnails – 660px X 420px
Portrait thumbnails – 420px X 660px
Square thumbnails -660px X 660px
Landscape large thumbnails – 1200px X 675px
So you can set larger thumbnails size 1200px X 675px
Best regards,
Shufflehound team