"Knights are rarely seen nowadays and are really more of an old Epic thing. They're single-man war machines that bridge the gap between dreadnought-sized walkers and titans. There are a number of different patterns, the two notable ones being the Paladin and Castellan.
Warhound scout titans are the smallest proper titans, and are used as a mobile reserve for the battle titans and as flank guards. Warhound titans mount main weapons systems in each arm, and have no point defense weaponry. Warhount titans have a crew of fiver-- a princeps, a gunner for each weapon, a comms operator, and an enginseer.
The lightest battle titans are Reaver titans, which are only marginally larger than warhounds, but have a more pronounced stance and feature a huge missile launcher on the shoulders in addition to arm mounted weapons systems. Reaver-class titans have a crew of six-- a princeps, a gunner for each arm weapon, a gunner for the missile rack, an enginseer, and a comms operator.
The main battle titans used by the Imperium are those of the Warlord class. They are slow, but feature weapons systems able to flatten cities in single volleys. They stand head and shoulders above Reavers, and are more than twice the size of a Warhound. Most Warlord titans feature automated defensive systems around the legs, and additionally have shoulder-mounted heavy weapons systems that are generally not as powerful as those located in the arms-- the shoulder weapons are generally comparable to what is carried by a Warhound titan. A Warlord has a crew of ten-- a princeps, a gunner for each arm and shoulder weapon, two enginseers, a comms operator, and a servitor hardwired to each leg's defensive weapons systems.
The largest titans fielded by the Imperium are the devastating Imperator titans, which tower over even Warlord-class titans. The deployment of such a war machine is a drastic measure, and many mount vortex weaponry. An Imperator Titan is no less than a mobile fortress, with butresses crewed by Skitarii armed with the myriad small arms common to the Imperial Guard. Depending on whether you count the guardsmen that fight from the butresses on the Imperator's legs, an Imperator titan can have a crew of several dozen. Integral to the titan's operations, however, are the princeps, gunners and loaders for the naval-class weapons mounted in the titan's arms, gunners for the shoulder and leg-mounted weapons, a full enginseer compliment, a comms operator, and a high priest of the Omnissiah to ensure that the most holy and potent of the Adeptus Titanicus' war machines are properly ministered to."
I might be confusing things, but I read something about a titan so big it had an air strip on each shoulder. Is that true?