Provided are a method for performing sidelink transmission by a first device (100) in a wireless communication system, and a device for supporting same. The method comprises the steps of: determining a transport block size (TBS) on the basis of whether at least one of an automatic gain control (AGC) symbol and a guard period (GP) symbol is used for sidelink transmission; and performing the sidelink transmission with respect to a second device (200) on the basis of the determined TBS, wherein the AGC symbol may be a symbol which a first device (100) uses for AGC, and the GP symbol may be a symbol which the first device (100) uses for TX/RX switching.