In CSS Grid, you specify the size and position of elements with grid-column and grid-row. Normally you specify the position with {start-position} / {end-position}.

But you can also specify the width with span:

#item {
  /* 3 columns wide, 2 rows high */
  grid-column: 1 / span 3;
  grid-row: 1 / span 2;
}

Check out this codepen example.