Provided are a method for a first device carrying out wireless communication, and a device supporting same. The method may comprise the steps of: determining the number of resource elements (REs) for determining a Transport Block Size (TBS) on the basis of the number of REs in a single slot and the number of REs on a Physical Sidelink Control Channel (PSCCH) resource in the single slot; acquiring an intermediate number associated with the TBS by multiplying the number of REs for determining the TBS, a code rate, a modulation order and the number of layers; quantizing the intermediate number; determining the TBS on the basis of the quantized intermediate number; and transmitting a sidelink (SL) to a second device on the basis of the determined TBS. Here, the number of REs for determining the TBS may be determined by excluding the number of REs on the PSCCH resource.