I’d really suggest to keep far below 51 bytes:
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In many, if not all, regions it is the maximum size for SF12 anyhow, so if the node needs to work in all situations, that’s your maximum.
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51 bytes allows for, for example, as many as 25 values between 0 and 65,536, or 12 values between 0 and 4,294,967,296.
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It might also be the maximum if you ever want to move to a different provider. Like the FAQ of a Dutch provider states:
What is the size of KPN LoRa packets?
The maximimum size that can be used all the time is 51 bytes. (When FOpts field within the LoRa packet is empty, this can be 54 bytes).Maybe “at all times” refers to the worst-case SF12, but this is just one example.
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With TTN you’ll also have to adhere to the Fair Access Policy.
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Even TTN has a reference in code to this maximum.
That said: the suggested changes work for others using LMiC, so without any further details how would you expect us to help?