This disclosure relates to techniques for reducing latency in a high-propagation-delay wireless communication system. A base station may receive from user equipment device (UE) a random access initiation message, having a characteristic that is configured to indicate to the base station a requested size of a subsequent uplink grant to be provided by the base station for transmission of uplink data by the UE. For example, the characteristic may include a predetermined preamble included in the random access initiation message, such that a requested grant size is indicated by which preamble the UE selects from a plurality of possible preambles. As another example, the characteristic may include which RACH occasion is selected by the UE to transmit the random access initiation message. In response, the base station may allocate resources for use by the UE based on the characteristic.