New cotton fitted sheet? Wash before first use and check shrinkage labels
Some new cotton fitted sheets specify washing before first use. Check the care label and shrinkage allowance, wash with like colors and dry as instructed.
IKEA's LEN cotton fitted sheet specifies washing before first use, a normal wash up to 60°C, no bleach and around 5% shrinkage. Those figures belong to that product; other sheets may have different limits.