How much data are you looking to send in the downlink?
In US915 the uplink and downlink datarates and limits are different.
The Rx1 and Rx2 datarates may be different. The Rx2 datarate is fixed and Rx1 can vary based on the uplink datarate and Rx1 Offset.
Class C will use the Rx2 datarate.
Maximum packet sizes for uplink and downlink are detailed in the regional parameters document.
https://lora-alliance.org/resource_hub/rp2-1-0-3-lorawan-regional-parameters/
53 bytes is the maximum payload at the lowest downlink datarate DR8 (SF12BW500) This can only be used if the Rx2 datarate is set to DR8 or the Rx1DROffset is increased above default of 0.
With default settings Rx1 will be at least DR10 (SF10BW500) and can carry up to 242 bytes.
The network may need to send MAC commands for ADR so allowing some bytes below maximum can help with network messages. An ADR command is 5 bytes.