A method and apparatus of a device that handles time-varying packet size in the downlink. In some embodiments, the method performed by a user equipment (UE) comprises: receiving a first signaling comprising configuration of downlink (DL) Semi-Persistent Scheduling (SPS), wherein the DL SPS configuration comprises a periodicity of the DL SPS and a first indication for a first physical layer resource, and wherein the first physical layer resource is used to dynamically modify size of a second physical layer resource for SPS DL data transmission; receiving a second signaling that activates reception based on the DL SPS, wherein the second signaling comprises information to specify a boundary of the second physical layer resource; monitoring the first physical layer resource on each SPS occasion based on the periodicity to decode a second indication; determining the size of the second physical layer resource for SPS DL data transmission, based on the second indication; and receiving data on the second physical layer resource determined for each SPS occasion based on the determined size.