Length of clearway ? - not quite.
Main limitation is an aircraft (AFM) one in that the takeoff run must be constrained to ensure that there is a low probability of the aircraft overruning the runway proper. For most rules, the maximum proportion of the airborne phase to screen height permitted over clearway is 50 percent.
The airport design standards give notional lengths for clearways but I am not aware that these are "limits" as such.