If a wall of water is more than 5 ft tall, could it be shortened to 5 ft to make it wider?
It would still be within the casting range of the caster and is really only meant to be an obstacle for medium and smaller creatures.
The 4th-level Sor/Wiz spell wall of water [conj] (Spell Compendium 235) is shapeable, which means the caster
can shape the spell. A shaped effect or area can have no dimension smaller than 10 feet. Many effects or areas are given as cubes to make it easy to model irregular shapes. Three-dimensional volumes are most often needed to define aerial or underwater effects and areas.
Emphasis mine. Further, the spell wall of water has an Effect that's
A straight wall whose area is up to one 10-ft. square/level (S)
Beyond this, the spell makes no mention of any other choices that can be made to resize or reshape it. For instance, there's no option to reduce its height to increase its length or make a dome of water.
That said, there's probably nothing wrong with a house rule allowing a caster to shape a wall of water more precisely unless, for example, the campaign involves a war against creatures from the Elemental Plane of Fire.