Once the total volume of an individual vessel has been established the detailed dimensions or mechanical design can be performed. Much of this is based on "Rules of Thumb"  built from years of experience. A summary is provided below  for a standard Stirred Tank Reactor (STR). 

Perhaps the most important factor in the mechanical design of a bioreactor is the height to diameter  or aspect  ratio  as this determines many of the other important bioreactor characteristics. There are relative advantages of increasing either the height or diameter.

(a) Increasing height

i.  increases the hydrostatic pressure

Advantage - increases solubility of oxygen

Disadvantage - increases solubility of other gases which may be toxic, e.g. CO2
 
ii.  allows more agitators

Advantage - better power development

Disadvantage - can lead to poor mixing due to compartmentalisation
 
(b) Increasing diameter

i. lowers the superficial (linear) gas velocity

Disadvantage - poor mixing, oxygen transfer
 
ii. reduces wall surface area (per unit volume) available for heat transfer

Disadvantage - leads to temperature control problems