Can someone tell me if there are FAA regulations on limits to size of aircraft that can be flown without copilot?
Thanks,
Yes. Someone can.
(most jets and most everything heavier than 12,500lbs)
Good answer you someone. . . 
MOST Airplanes over 12500 lbs will need a crew of 2 or more but some can still fly with just 1 as long as you have the proper equipment and rating to do so (see FARs), a Cessna 560 Citation V or Encore can be single pilot and take off at 16500lbs but a old lear at 10000lbs might need 2 pilots and the C560 without the proper rating would also need 2 pilots.
So you would need to be a little more exact about what plane.
MOST Airplanes over 12500 lbs will need a crew of 2 or more but some can still fly with just 1 as long as you have the proper equipment and rating to do so (see FARs), a Cessna 560 Citation V or Encore can be single pilot and take off at 16500lbs but a old lear at 10000lbs might need 2 pilots and the C560 without the proper rating would also need 2 pilots.
So you would need to be a little more exact about what plane.
That was such a good answer that it needed repeating.
Thats basically what I said in my response
“most jets and most everything over 12,500lbs”
Maybe nobody reads the fine print anymore. 