What is the maximum body weight allowed for a pilot?
In the US there's no specific weight limit in the regulations. Instead, they just say that you must be capable of performing the duties of a pilot, e.g. 14 CFR 61.113:


  The general medical standards for a first-class airman medical
  certificate are:
  
  [...]
  
  (b) No other organic, functional, or structural disease, defect, or
  limitation that the Federal Air Surgeon, based on the case history and
  appropriate, qualified medical judgment relating to the condition
  involved, finds—
  
  (1) Makes the person unable to safely perform the duties or exercise
  the privileges of the airman certificate applied for or held; or
  
  (2) May reasonably be expected, for the maximum duration of the airman
  medical certificate applied for or held, to make the person unable to
  perform those duties or exercise those privileges.


But the FAA is concerned about some conditions that are strongly linked with obesity, like heart disease, diabetes or sleep apnea. Those could be an issue for getting a medical certificate.

Apart from the regulations, there may be some practical weight limits for other reasons:


Military pilots have much stricter medical requirements (I know you're asking about commercial pilots), partly because they have some special considerations
A very large pilot may be physically unable to fit into some cockpits and/or move the controls to their limits
A very heavy pilot may be too heavy to fly in some smaller aircraft (including training aircraft) because of weight and balance limitations