Dear Pankil,
We have done tests of distance with 20-byte packets achieving the 1% PER at several distances. However, the results vary depending in several factors such as line of sight, antenna/board orientation and antenna type. Assuming you are using TWR-KW2x boards at best antenna orientation, you should be able to get the 1% PER up to around 150 meters.
If the payload is increased, the PER is somehow affected since the more bytes need to be sent, the more susceptible is to noise or interference. Since the PER will evaluate the correctness of packet reception, any change in it will result in a bad packet. Therefore, while it might not be formal relationship between distance and packet size, a larger packet would affect the distance indeed.
In terms of testing, the PER would definitely help you understanding and setting the limits of your application. You could make some measurements with your current bards and get the relationship between packet size and distance, along with board orientation metrics. SMAC connectivity test, in general, is recommended for radio testing purposes being a simple yet useful tool.
Regards,
AngelC
