Answers for "css grid template is not fullsize"

css grid full width row

.child-item {
  grid-column: 1 / -1;
}

css grid not taking full row height

.img_container{
    height: 50%; // or whatever value just make sure you give your parent a height
    width: 100%;
    background: url('link to your image') 50% no-repeat;
    background-size: cover;
 }