Actually perhaps “leadholder” is a more appropriate term than “mechanical pencil” as most, if not all sketch pencils are clutch mechanism pencils.
I’m no artist and I don’t really have many sketch pencils, but here are a few photos of two that I do have.
The Koh-I-Noor ‘Versatil’ 5347, 5.6mm clutch pencil
You can see how thick the 5.6mm leads are, pictured here with an ordinary wooden pencil. They are huge!
Some 3.2mm Kaweco leads – graphite and coloured. There doesn’t really seem to be a lot of coloured leads available, certainly not compared to ordinary wooden coloured pencils.
5.6mm and 3.2mm pencils.
The Kaweco “Sport” bunt 3.2mm clutch pencil is short and wide, of octagonal cross-section. It is 104mm long x 13mm wide across the flats, that’s about 4 x 1/2 inches.
Now, for the artist who likes a pencil with a stratospheric price tag, there is the Montblanc Leonardo Sketch Pen (it’s a pencil), as reviewed over at Paper and Pencil.
“Ladybug” and “Butterfly” in graphite and coloured pencil, by one of my young relatives. The Leonardo is definitely beyond her price range, but if she grows up to be a famous artist…