A container with 18 gallons of capacity would hold approx. 75 liters. There are varying dimensions which could produce this capacity.
In their book, The Bountiful Container
, Rose Marie Nichols McGee & Maggie Stuckey recommend a minimum depth of 6 inches (15 cm) for containers in which lettuces and other salad greens (mizuna, arugula, radicchio, etc.) will be grown.
As to the spacing of the romaine plants, envision the size of a full-grown plant. Now give it some breathing room--which can also function as maneuvering room for your hands--and leave space for the next romaine plant to grow also. This distance will become your spacing between plants.
I grew lettuce last year and used the "cut and come again" technique, so my lettuce plants never achieved full size; their outer leaves were always cut as new inner leaves pushed the outer ones to the edges.
So I cheated a bit on the spacing and put them slightly closer together than most gardening authorities--even those who advocate intensive methods--would have recommended.
Sunset Zone 17, USDA Zone 9