What's the MTOW I can fly with a PPL? I thought there was a limit but I can't remember and can't find it anymore.

So with an US PPL, what would be the maximum takeoff weight that an aircraft would be allowed to for me to still fly it?

It'd be great if anybody could point out the appropriate FAR, too!
There are no explicitly regulated MTOW limits for a private pilot with a valid medical but extra certifications/training may be required to fly heavier aircraft. 

The often cited number is 12,500 but this is somewhat incorrect depending on how you look at it. You will need a Type Rating to fly anything over 12,500 but strictly speaking those can be added to a private pilots license. 

If you are a PPL flying under Basic Med you are limited to 6000 LBS MTOW


  14 CFR § 61.113 - Private pilot privileges and limitations: Pilot in command.
  
  (i) A private pilot may act as pilot in command of an aircraft without
  holding a medical certificate issued under part 67 of this chapter
  provided the pilot holds a valid U.S. driver's license, meets the
  requirements of § 61.23(c)(3), and complies with this section and all
  of the following conditions and limitations:
  
  (1) The aircraft is authorized to carry not more than 6 occupants, has
  a maximum takeoff weight of not more than 6,000 pounds, and is
  operated with no more than five passengers on board; and




Just a note on practicality, even if you don't get into the 12,500 range you are likely to need high performance and complex ratings to fly anything much bigger than a 172. While there are some big fixed gear aircraft out there most of your common 6 seat planes (Saratoga, Bonanza, etc) are retracts.