max-width for the container they're in and stating the sizes using fractional units and percentages in grid-template-columns/rows etc to get the sizes you need or is it a matter of defining sizes of the divs using width/height? flex property on the divs or setting the first divs height in percentage to take up more of it's container e.g. height: 70%;flex: .6 set on it and the div on the right having flex: 1; set on it. the width of the image box class is 33%, i can see why this is done as 33 x 3 is 99 so almost 100% of the container allowing the sizes of the boxes to be even with a flex-wrap: wrap set on the div holding them to bring them to the next level;

