The barrier around the palace is the same basic construction as the one surrounding Tu Vishan himself. So it would stop Cassie, but not because of her size, it's sensing her life energy and that's what would prevent her.

(That said since it's possible to leave the shell on various missions or when accompanied by Han, etc, theoretically it is also possible to breach the Palace barrier too, and something in that vein may be part of this month's missions, yes indeed.)