First, there is a vague description in the book about the size, type, and purpose of each Gomer Ship encountered. On a battlefield, this is about all you have to go on, . . . . at first. There are at least two more books in this series, and there will be more answers as each book unfolds. Like a stripper, would you rather they came out stark naked, or would you prefer to be teased?
I also like to let people use their imaginations to “see” the threat. Call me old school, but of the two versions of the movie Psycho, the first one is far more frightening to me. Sometimes it is the horror you don’t see that will trigger your own imagination as the reader. I assume that the average science fiction reader already has a vivid imagination, and as someone who can dream within the realm of science, you can create something far more frightening to you than I ever could in a million years.