An image reading apparatus, comprising: a platen having a transparency and on which a sheet is placed; a support configured to support the platen; a reader configured to read an image by scanning the sheet placed on the platen in a first direction; open/close means configured to be able to open/close with respect to the platen, and comprising a pressure plate that presses the sheet placed on the platen when in a closed position with respect to the platen; open/close detecting means configured to detect whether the open/close means is in the closed position with respect to the platen, wherein a state in which the open/close detecting means detects that the open/close means is in a closed position with respect to the platen is a first state; first obtaining means configured to, in the first state, obtain information related to a position of an edge positioned outermost in a second direction of the sheet based on a reading result of the reader, the second direction being a direction orthogonal to the first direction; a mark formed on a pressing surface, which presses the sheet, of the pressure plate; a positioning member configured to align the sheet placed on the platen in the second direction by contacting one edge of the sheet; determining means configured to, in the first state, determine that the sheet is not protruding from a readable region in the second direction in a case where the reader reads the mark, and determine that the sheet is protruding from the readable range in the second direction in a case where the reader does not read the mark, the readable region being a region, on the platen, where the reader is able to read the image, and second obtaining means being a sensor arranged at a position at which it can distinguish between two original sizes for which a length in the second direction is the same but a length in the first direction is different, wherein a position of the mark is included in a region on the pressing surface corresponding to the readable region and is provided at a position at which, in a state where a sheet of a maximum size among standard sizes is aligned with respect to the positioning member, a portion of the mark is not hidden by the sheet of the maximum size as viewed from the reader, a length of a sheet of the maximum size in the first direction being shorter than a length of the readable region in the first direction and a length of a sheet of the maximum size in the second direction being shorter than a length of the readable region in the second direction, and wherein the determining means is configured to determine a size of the sheet based on a result of the determining means, the first obtaining means, and the second obtaining means. a platen having a transparency and on which a sheet is placed; a support configured to support the platen; a reader configured to read an image by scanning the sheet placed on the platen in a first direction; open/close means configured to be able to open/close with respect to the platen, and comprising a pressure plate that presses the sheet placed on the platen when in a closed position with respect to the platen; open/close detecting means configured to detect whether the open/close means is in the closed position with respect to the platen, wherein a state in which the open/close detecting means detects that the open/close means is in a closed position with respect to the platen is a first state; first obtaining means configured to, in the first state, obtain information related to a position of an edge positioned outermost in a second direction of the sheet based on a reading result of the reader, the second direction being a direction orthogonal to the first direction; a mark formed on a pressing surface, which presses the sheet, of the pressure plate; a positioning member configured to align the sheet placed on the platen in the second direction by contacting one edge of the sheet; determining means configured to, in the first state, determine that the sheet is not protruding from a readable region in the second direction in a case where the reader reads the mark, and determine that the sheet is protruding from the readable range in the second direction in a case where the reader does not read the mark, the readable region being a region, on the platen, where the reader is able to read the image, and second obtaining means being a sensor arranged at a position at which it can distinguish between two original sizes for which a length in the second direction is the same but a length in the first direction is different, wherein a position of the mark is included in a region on the pressing surface corresponding to the readable region and is provided at a position at which, in a state where a sheet of a maximum size among standard sizes is aligned with respect to the positioning member, a portion of the mark is not hidden by the sheet of the maximum size as viewed from the reader, a length of a sheet of the maximum size in the first direction being shorter than a length of the readable region in the first direction and a length of a sheet of the maximum size in the second direction being shorter than a length of the readable region in the second direction, and wherein the determining means is configured to determine a size of the sheet based on a result of the determining means, the first obtaining means, and the second obtaining means.