Sections – details and changes

Thanks for the beautiful theme! I would like to ask you what is the suitable dimension for the images used for the boxes in Sections? I did not find any information in the documentation. I was playing with different image dimensions a while but I am still not satisfied. Also, I am interested in rewriting "Read more" text in the buttons to anything else. Is it possible to do in function.php of the child theme of the parent Charity Help theme? Thank you in advance.