You have not said anything on where and what the function of reactor. I presume you are using reactor either for chemical or heat transfer processing. Usually 1st order exothermic reaction takes place in plug flow reactor (PFR) that has pressure drop and heat flow through the wall. You need to carryout demonstrations by varying the reactor diameter keeping the total feed flow rate constant in the reactor in calculating the pressure and temperature drop in order to calculate the required diameter of the SS tube. Also look at the paper " Effect of tube diameter on PFR" by Raphael Bowman in 2014 using the website - www.demonstrations.wolfram.com This would help. Good luck.