P2P mode max payload size

How many bytes can the RAK11720 module transmit via LORA P2P at most?

Max payload is 250 bytes.

hi beegee ?
Is it possible to increase the payload size?

Not without changing the LoRa drivers.
By default the internal buffer of the LoRa transceiver is setup to 1/2 half RX buffer and 1/2 TX buffer.

Is it possible to send data every 200 ms using the api.lora.psend() function? I changed the spreading factor to 7, but it still seems to successfully transmit only about once every 3 seconds.

LoRa is not a high speed transmission medium.

Check Semtech LoRa Calculator to find out how much time a transmission takes.

SF is not the only factor, it depends as well on bandwidth, preamble-lenght, and coding rate.

220ms transmission time with 250 bytes, 125kHz BW, SF 5 and CR 4/5