So here's my take on what pb&j look like:
Disclaimer: I think before i start i need to make clear that this is my own (personal) take and i love all depictions of them (all the love to my fellow artists and writers <3).
As a child he was probably/definitely overweight and not very tall compared to his peers (which, together with his nerdy personality, earned him his various nicknames) but as he grows he grows taller and he grows into his weight (if that makes sense). Essentially he comes to fill out his body with more muscle but still has a healthy amount of body fat. He has broad shoulders and thick thights. His arms are strong which makes him such a good swimmer and at some point he becomes so strong he could beat Peter during arm-wrestling every time.
He has dark hair, either dark brown or black and brown eyes (puppy dog eyed). He has long dark lashes and tan skin (mostly his genes but also) from working in the sun all day. He has thick/dark eyebrows and full lips. His face is round and his features defined. His nose has a broad bridge. He has a nice ass (sue me)
He puts a good amount of hair-gel in his hair to keep it out of his face, because it otherwise tends to bother him when helping Titus.
He wears flannel-shirts over t-shirts/tank-tops and loose fitting jeans. Most of his jeans have some kind of stain (oil, dirt that cant be washed out anymore) or little holes from getting caught in bushes during observations.
His nails are always tidy and clean (somehow even after hours of working outside).
Personally i imagen that he has some kind of south american ancestry. Maybe his dad was (half) mexican.
He is incredibly tall and as a teen lanky. With age he starts to fill out his body more and where muscle was visible before he becomes incredibly defined. He has long legs that seemingly go on for days. His thights lool like where Justus is all intimidating strength Peter is stealth and agility. He still runs and still surfs with a passion. He still plays basketball and he is still incredibly good at any sport he tries. He's tan as well tough his tan is not as natural as Justus'. Peters tan comes from spending all day every day in the sun. His hair is a rusty brown (brown with a hint of red, more orange-y maybe). He has green eyes that have gotten him more compliments than he cares for. Not light green more of a dull olive.
His eyebrows are thin (he or one of his older sisters probably plucks them regularly) and when he thinks hard a 'Falte' appears between them.
He has well defined abs and truly impressive chest muscles (he can move them and likes to annoy Bob with it whenever he can - he calls him a "show-off")
His hands are large and he loves to hold hands with anyone that will let him.
Hid forearms are well-defined with visible veins.
He has freckles ALL over his body. The further down you go the fainter they grow but they are there none the less.
Fun Fact: he always makes sure to wear a lot of sunscreen and regularly reapply.
He is the smallest high-wise of the group but is still taller than you'd expect (maybe like 1,79 m cause for some reason 1,80 feels to tall).
He has blonde wavy (not quite curly) hair that he wears as a mullet. And mire of a "Straßenköterblond" not a light or honey blonde. He has blue eyes that look slightly different depending on the lightning and the colour of his clothes. If he wears something dark blue his eyes will look darker and if he wears something green they will look more teal.
He has long fingers and always clean cut nails. He occassionally likes to wear nail polish but only to special occassions. Every few months he gets an itch and either he'll cut his hair or get blonde highlights.
He is also kinda tan but tends to get sunburnt more easily than thr others.
He wears square glasses with a brown plastic frame and he used to have an eyebrow piercing.
He also has a tattoo that Justus/Peter don't know about. (It's a questionmark behind his ear).
His stature is more muscular/defined than is often depicted.
He helps Titus out a lot as well and after all their cases he has taken up self defense classes which helped him build a stronger physique. Regardless he is often underestimated especially strength-wise because his build doesn't really fit the athletic (Peter) or dark&broody (Justus) type.
His waist is thin and he has somewhat of a sleeper build. Peter & Justus had to learn the hard way not to underestimate his build.
(He can carry Peter through the forrest for an hour.)
They all have their fairshare of scars.
Bob is attractive in a Chris-Evans-kinda-way, everything looks good on him. Glasses, no glasses, long hair, short hair, any haircolour really, pants, skirts you name it.
Peter is attractive in a hot-athlete kinda way. All tall and lean muscle. Straight out of a surf magazine or a sports ad.
Justus is attractive in a dark and broody, mysterious quiet guy kinda way. Where he was timid as a child, he now has a strong presence, that makes people perk up when he enters a room.