Why not "Tall ditch"? Is there any other way to say "tall ditch"? I do not know much yet.
"Tall" is generally up. A "tall" ditch would normally be called a deep ditch, not a tall one (unless, maybe you were an ant at the bottom, looking up).
In English you have a lot of words for "big" things. Tall is used for height only. You would say a tall man, not a long man. Roads, canals, railways are long not tall. With words like big, large and great you have similar differences. Large is used for surfaces, big for three dimensional size, great is used for for non physical size Bach was a great composer