A twin fitted sheet is 39 by 75 inches, while a twin flat sheet is 66 by 96 inches. Sheets for an extra-long twin bed are the same widths, but the lengths are 80 inches long for the fitted sheet and 102 inches for the flat sheet.
One dimension that standard sizing charts often don't mention is the depth of the mattress, which can range from 10 to 18 inches. The best fitted sheet for a mattress is the one that fits it most closely along all three dimensions. For flat sheets, the width is the most critical dimension, as excess length can be tucked under the foot of the mattress.