Aha! This is an old thread but I ran into this problem today and solved it to my satisfaction by combining two of the CSS bits provided, so that I get only 3 related items suggested AND they are not overly large. Thus:
.ProductItem-relatedProducts .list-grid .grid-item:nth-child(n+4) {
display: none;
}
.ProductItem-relatedProducts .list-grid
{grid-template-columns: 200px 200px 200px 200px !important;
}
Hope this helps someone! I know I will be making a note of this for future use. ^.^
