There is a hard-coded MQTT packet size limit on an underlying library that the Arduino client uses. Check out this post and see if it addresses the limit issues:
If you’re using a Losant Builder Kit or the losant-sdk-arduino on another board, you’ll likely hit the default mqtt packet size limit defined in the underlying pubsubclient library. Unfortunately the packet is blocked before reaching any of your code, so it’s hard to debug. It simply looks like the command was never received. For example:
{ "foo" : "bar" } works, whereas { "somethingLarger" : "with a longer value" } doesn’t work. This is because the default packet size is 128, which provides en…