An image forming system operable to perform image formation on a sheet includes a storage for storing a sheet, a conveyor configured to convey a sheet along a conveyance path a sheet size setter configured to set a size of the sheet stored in the storage, a sheet size detector configured to detect a size of the sheet conveyed by the conveyor, at least one memory that stores at least one program, and at least one processor. The at least one program causes the at least one processor to perform obtaining a difference between the size set by the sheet size setter and the size detected by the sheet size detector, and controlling image formation, wherein when the difference is greater than a first reference value, the processor interrupts image formation, and when the difference is less than or equal to the first reference value and greater than a second reference value, which is smaller than the first reference value, the processor notifies a warning without interrupting image formation.