The Amis are to be found from Marius Book 4: The Friends of the ABC (pg 646 of the Fahnestock McAfee) to Jean Valjean Book One: Orestes Fasting, Pylades Drunk (pg 1253).
-austere face, deep eyes with slightly red lids, thick underlip (readily disdainful), high forehead; fair lashes, blue eyes, rosy cheeks, exquisite teeth; pale
-tall, slender, blond (Navet mentioned Enjolras as "a big fellow with blond hair"; elsewhere in the book it is mentioned that he had "fair hair")
There's very little physical description (if any!) on the rest of the Amis.
-[not canon, but given that he was based on Desmoulins, I tend to think of him as the shortest of the lot]
- dressed badly, usually kept his head and eyes down, prone to blushing
-Hugo said that Courfeyrac "had all the qualities of a centre - roundness and radiance". I choose to take it literally to mean that he was chubby!
-wore loud waistcoats. He wore a red one to Lamarque's funeral and the insurrection
-nothing mentioned about his build, but it's unlikely he would've been willowy if he liked fighting all that much!
-cheery, bald (I don't know if Hugo simply meant that he had nothing, or if he was literally missing hair; many people take it to mean the latter)
-wore threadbare coats
-carried a cane, was forever rubbing his nose with it
-his build is not specifically mentioned, but this guy knew boxing, tennis, dances and was a "knowledgeable cudgel player", so he was probably in decent shape