(a) I'm not aware of any hard standards on maximum stopway/clearway lengths (perhaps one of our airports engineering specialists may care to offer some expertise on this point ?) but guidance is provided in documents such as FAA AC 150/5300-13 which suggests a practical clearway limit of 300m at pp23.
(b) From the point of view of a particular aircraft, the maximum clearway able to be scheduled is determined by the TORR and TODR. Depending on the rules applying, a proportion of the airborne distance from liftoff to screen (usually one-half) is required to be scheduled over the physical TORA declared for the runway. This, naturally enough, is very desirable to give the aircraft some chance of getting off the ground before the end of the smooth bit .....
If the takeoff is TOR-limited, the provision of excessive clearway is unusable and of no scheduling use for the operator ... all clear area, however, is gratefully appreciated ...