In World War I trenches were big business; neither side could advance effectively, and so they dug in to ensure that at least the enemy's artillery couldn't do too much damage. Networks of trenches stretched for hundreds of miles, containing bunkers, kitchens, supply dumps, and anything else needed for daily life in war and under bombardment.
In World War II trenches were less common; air support, blitzkrieg tactics, and highly mobile armies limited the use of large trench networks in Europe. But there were still lots of trenches, as getting underground is a really effective way of not getting blown up. Even very mobile armies tended to dig in when they got to the front, and journalists, soldiers, and historians wanted to describe what they were doing. The foxhole remained a common solution, and were familiar to the public. Slit trenches were less familiar -- and today are even less so -- but quickly came into common parlance. A slit trench is what most people probably think of when they think of a trench: it is a long, narrow, ditch that is deep enough to hide a person when standing.
In 1942 a slit trench was a trench about 6 to 7 feet deep, with a fire step about two feet from the bottom, allowing troops to get high enough to fire over the edge. They were generally about 6 feet in width. Of course, these were constructed in a hurry and in a wide range of geomorphological and geopolitical conditions, and they were often abandoned just as quickly, so expecting exact dimensions is not reasonable.