There is also two others:
Sons of Dorn - which gives a description of the main docking bay - there's a single primary bay which (internally!) docks rapid strike craft and small craft, and which is (obviously) huge.
It has a huge relic chapter banner hanging from the walls, which are changed to commemorate significant victories (the story follows a scout - during his entire tenure in the 10th, none of the battles fought by the chapter counted as 'significant'....), alongside one banner for each company (each about 16' square but small in comparison!).
The current one is Black-on-Yellow, with a motto of 'VII', 'Roma', and 'Sons of Dorn' (after the fist's first, unification-war era battle honour), and older transports with famous histories suspended from the ceiling or walls as trophies and monuments. The book also mentions duelling arenas (more elegant swordsmanship with combatants having their feet restrained in duelling blocks, not world eater/ork style "fighting pits")
Phalanx, from the Soul Drinkers series, is unsurprisingly set mostly on the Phalanx. Aside from prison cells, the three areas most mentioned in detail are an arboretum garden (directly or almost directly adjoining the trial hall), the hall where the trial takes place (a sort of open Grecian Amphitheatre able to hold a representatives of a dozen chapters at once, with an armourglass dome allowing a view of the stars), and - randomly given what you're talking about - a hall of dioramas, recreating famous battles fought by the fists, primarily from Rynns World - there is a Rynns World memorial - a stone amphitheatre with names of every crimson fist lost. The 'Forge of Ages' is essentially the armoury deck.
Externally....There is the cover artwork for Phalanx, but that is to me pretty clearly a Ramiles class star fortress. still,
tt's possible the Phalanx is such an animal, just mobile. To my knowledge there isn't a detailed external description - although there may be one in the
HH novel Flight of the Eisenstein somewhere.
The cover art: