OK after someone made a comment about how small most DropShips are I'm looking at putting pen to paper for a design that I've been kicking around for some time: A DropShip that can lift an entire regiment at once. Now SO says that a regiment consists of 132 'Mechs maximum, but the Lyrans added an extra Company during the 4th SW brings it to 144, add 36 ASF for fighter screen for total bays needed to 180 and I'm rounding up to a nice round 200. So that many bays weighs 30,000 tons. So I'm probably looking at a minimum weight of 50,000 tons.
So questions:
1) The big one: How many guns should I stick on this thing to protect it?
2) Could I get away with thrust of only 2/3 or do I need a minimum of 3/5?
3) How much SI?
I'd be FAR more interested in something that moves a Regiment of Infantry v/s a regiment of Mechs w/ attached Fighters.
But, since that isn't what you asked.
Use the Mammoth chassis IMHO for ease of design.
Maybe call it the "Mastadon" variant for combat transport.
Of the 144 Mechs, I'm with Colt, a lot of them don't have to be droppable from orbit, save some tonnage & carry a bunch of them as Cargo.
I'd convert one of the cargo bays over to 52 Mech Cubicles allowing a reinforced battalion to drop from orbit & secure the LZ.
Convert another over to your 18/36 Fighter Bays & an infantry battalions worth of bays for all the pilots/techs/command staff, etc etc.
The rest leave as cargo space for more mechs, tanks, or you know, cargo :)
For the chassis itself, ARMOR ARMOR ARMOR, Max it out in case its attacked.
You can pump up the SI some but I wouldn't go crazy there.
Its already 20 so I'd leave it at 20 or if you REALLY want more, increase to no more than double (40)
Depending on the era, I'd add MG/SL, SPL/AMS, &/or Screens for protection.
Finally give it some basic guns to fill out each arc to the soft cap of 12.
Paired LLs/MLs, LRM 15/20 Racks, PPCs, or more advanced ER/Pulse/Gauss/LBX are all options.