The disclosure relates to an image forming apparatus. An image forming apparatus includes a stacking portion on which a sheet is to be stacked, an image forming unit, a size detection unit, a setting unit, a notification unit, and a control unit. The image forming unit forms an image on a sheet fed from the stacking portion. The size detection unit detects a size of a sheet stacked on the stacking portion. The setting unit sets at least one sheet size to be allowed for use at the image forming apparatus. The notification unit provides notification of information. The control unit causes the notification unit to provide a warning in a case where the size of the sheet detected by the size detection unit is not included in the at least one sheet size set by the setting unit.