For an adult circumcision, you are better off getting a circumcision that is tighter (within reason) than you'd want.
When done as a child/baby, every growth spurt helps normalise the circumcision to the right length. If done too lose, the glans grows out of the skin. If done tight, the skin strethes to accomodate the growth.
And during puberty, frequent erections and the homones that foster skin growth combine to stretch the skin to accomodate growth (if necessary).
When cut as an adult, if cut too loosely, the skin will not "shrink to fit" because you aren't growing anymore. But if cut tighter, the skin will loosen back over the first couple of years to be of just the right length.