MSP430FR6043: USS gas pipe design

Hi,

I use MSP430FR6043 along with 200k/500k transducers to measure gas flow.

I have some questions regarding the pipe design.
I wonder what are the rules for finding the two basic parameters:
1) pipe diameter
2) distance between transducers

ad1) What is the desired maximum gas velocity we must take when calculating the pipe diameter?
On the one side, a higher maximum velocity means higher measurement resolution. But, on the other hand, higher velocity means more pressure on one of the transducers, non-linear flow, and possibly a dozen other factors.

ad2) What is the proper way to find the desired distance between transducers (length of the up and downstream traveling)?
On the one side, a longer distance means longer time and, therefore, better resolution. On the other side, a longer length means higher attenuation and worse SNR.

Regards
Adam