I suppose Din buffer could/should have same size on 868LP (is it on manual?)
Is not clear if Din and Dout buffers are separated, I suppose yes.
So, if I try to send API1 message I need the TOTAL message length would be 202-FT value.
Is there a command to check Din buffer size BEFORE start transmission?
The command you want to query to get the max data packet you can provide in your API frame is the NP. The NP command is designed to provide you with the maximum data payload size supported by the product and its related firmware.
Oh and yes, there are different buffers for DI and DO. Generally DO is larger than DI.