Cold Smoker Chimney Size

Hi All,  I have a two box cold smoker.    The first box I use to supply the smoke to the larger smoking box (3.5' x 3' x 1.2'), using a 3.5" transfer pipe.    On the small (1.5' x 1.5' x 1.5')  fire box I use a 12" pellet smoker tube and have 4 individual 2" diameter inlet holes.    These can be opened or shut independently and I usually have just the one opened in front of the pellet tube opening.    Now my problem...I have a 3.5" chimney about 1.5' long and am thinking of replacing it with a 2" chimney which is a tad longer.    My question is: What is the optimum size and length when using a 2"-2.5" inlet?    I'm thinking the 3.5" doesn't allow enough drag and my pellet tube goes out.    I have tried various open vent combos on the induction, but unless there is a good breeze it goes out.    Does a too fat chimney suck as well as a smaller one and what is the optimal length?   Hopefully someone will have some good advice. Cheers Oscar