You'll pass two huge sentinel statues 21 m (68 ft) high on the river plain facing out towards the Nile.
The two sentinel statues are 21 m high and 3 m wide. 
mnli hypothesis: The two sentinel statues are 21 m high and 3 m wide.  premise: You'll pass two huge sentinel statues 21 m (68 ft) high on the river plain facing out towards the Nile.