@Lesum wonderful, thank you!!
That worked, and I altered the code a bit so the related product sizes on desktop view would still be smaller, in a row of 4 columns. In case it helps anyone else, here it is--
@media only screen and (max-width: 767px) {
#pdp .ProductItem-relatedProducts .list-grid {
grid-template-columns: 1fr 1fr !important;
grid-column-gap: 10px !important;
grid-row-gap: 10px !important;
}
}