Off the top of my head, I think a 200-ton shuttle can fit 100 tons for two Light Vehicle Bays plus enough infantry to be carried by those vehicles. Conventional Foot to BA works here, I think... 
More or less.
As a barebones example, here's the LCM(A), or Landing Craft Mechanized (Aerodyne). Just enough thrust to conduct both landing and recovery operations, just enough armor to see it through contested space (assuming there are escorts for the craft to deal with most of the resistance). No defensive weapons. The cargo capacity is divided into a 100 ton "troop deck", which could be configured for two light vehicles, two points of protomechs, or two stars of elementals. Or a mix of those and general cargo. The second, smaller bay is available for general cargo but could be configured for two to three platoons of infantry depending on the type. Heck, if the flight is short enough duration, you could probably have it configured as an infantry compartment and pack a hundred and sixty guys in there.
LCM(A)
Type: Military Aerodyne
Mass: 200 tons
Technology Base: Inner Sphere (Standard)
Introduced: 3120
Mass: 200
Battle Value: 580
Tech Rating/Availability: D/X-X-D-D
Cost: 6,575,500 C-bills
Fuel: 10 tons (800)
Safe Thrust: 3
Maximum Thrust: 5
Heat Sinks: 0
Structural Integrity: 5
Armor
Nose: 72
Sides: 52/52
Aft: 52
Cargo
Bay 1: Cargo (16.0 tons) 1 Door
Bay 2: Cargo (100.0 tons) 1 Door
Ammunition:
None
Escape Pods: 0
Life Boats: 0
Crew: 1 officer, 2 enlisted/non-rated
Notes: Mounts 13 tons of standard aerospace armor.
Weapons
and Ammo Location Tonnage Heat SRV MRV LRV ERV
None
A spheroid hull would give you more space to work with (and make it slightly easier to mount weapons since it will actually have free heat sinks) but it would be less agile in the atmosphere.
