While sheet sizes vary by brand and style, a fitted sheet is similar to its respective mattress in measurements; for instance, 78 inches wide and 80 inches long for a king mattress, compared to 72 inches wide, 84 inches long for a California king. The fitted sheet has a little extra width in the king-sized set. Flat sheets are larger: up to 109 inches wide and up to 106 inches long for a king, compared to up to 108 inches width and length for the California king. Flat sheet sizes vary greatly from one style to another.