@TherealKate as @anon4126930 said there is no maximum robot weight requirement spelled out by the game manual.
However, your choice of the drivetrain type and gearing will generate a number that is practical.
For example, if you have four 4" omni wheels on your base directly driven by 200 rpm V5 motors they could generate about 18 lbf of the force, which is about the same as 19.5 lbf of the ground friction for 15 lb robot on those 4" omnis.
Lighter robot will not be able to take advantage of all the pushing force those motor provide and heavier robots will just waste more power on friction without giving you more pushing force at 200 rpm direct drive.
Most of the VRC robots naturally end up in the range of 13-17 lbs with a few exceptions lighter or heavier. (I didn’t do formal study, I just keep asking people what is the wight of their robot all the time 
)
If anyone build such a robot they need to consider changing their drivetrain gearing or number of drive motors such that pushing force better matches the tractive friction and friction losses in the drivetrain.