The current end-device datarate is provided with the uplink.
Downlink datarates use DR8-DR13 in US915
Response to uplinks using DR0-DR4 in Rx1 use DR10-DR13 which all allow 242 bytes payload.
DR8 allows 53 bytes and is default Rx2 datarate. Rx2 datarate can be set at network server to DR10 and the setting will be transmitted to the end device on OTAA join.