Constantine is based on the British musician Sting. He's about 6 feet or 6'1" tall, has blonde hair, smokes like a chimney and always wears a trenchcoat. He usually has a look of world-weariness that implies that he's seen more hard times and hell than any ten men have ever seen.
Here are a few photo references of Sting:
|Younger days. This is how Steve Bissette drew him in Constantine's first appearances.||Good view of his facial structure. I imagine Constantine as much more gaunt than this - he's a big drinker and smoker.||Constantine often is unshaven and rather unattractive.|
|This might be the best reference for Constantine's look, IMHO. This is more the look of world-weariness that I see in Constantine.||Sting's smile here is very Constantine.||This is more the body I see the character having.|
And here are some illustrated references for Constantine: