This is a tricky balance. Too much description and your reader might tune out. Too little and you risk not presenting the character the way you intended. A good practice is to choose a few defining details, ones that capture the essence of the character’s appearance. This way, you fix the important details and readers can still actively engage their own imaginations, filling in the rest. For example, in Annie Proulx’s The Shipping News, Quoyle is described like this:
A great damp loaf of a body. At six he weighed eighty pounds. At sixteen he was buried under a casement of flesh. Head shaped like a crenshaw, no neck, reddish hair ruched back. Features as bunched as kissed fingertips. Eyes the color of plastic. The monstrous chin, a freakish shelf jutting from the lower face.
At this point, my version of Quoyle may have attached ear lobes and a sprinkling of moles on his arms and yours may not, but it doesn’t matter, because Proulx has laid out the essentials of Quoyle’s appearance. We both have the same idea of how Quoyle inhabits space, from his size to his “monstrous chin.” (In fact, Quoyle often covers his chin throughout the novel. It becomes his most defining physical feature.)
Details of appearance don’t need to be so prominent—or monstrous—to define. Terron Musgrave, a character in Russell Banks’ The Book of Jamaica “wore his hair in long, matted, leonine locks called dreadlocks, and in profile he did indeed resemble a dark male lion, which was as he desired it.” Notice how this detail also gives insight into the character. In fiction, attention to appearance shouldn’t just create the outer shell; it should give a glimpse inside, too.
Don’t be afraid to use metaphor or simile when describing appearance. It can be an effective and artful way to convey both a bit of the outer and the inner, as in this description of a grandfather who is almost a hundred years old in Tayeb Salih’s Season of Migration to the North:
He is no towering oak tree with luxuriant branches growing in a land on which Nature has bestowed water and fertility, rather is he like the sayal bushes in the deserts of the Sudan, thick of bark and sharp of thorn, defeating death because they ask so little of life.
And remember, you don’t have to get all the details of appearance in one big chunk of narrative. An accumulation over the course of the novel or story can create a full picture of the character, and if the details are attached to meaningful action, the reader won’t even notice how it’s happening. A bit like sneaking the dog’s medicine in a piece of tasty liverwurst.