The mecha in Modular Mecha (Pyramid volume 3 issue 51) are fairly small (17-33 feet tall and 25-65 tons) compared to those seen in some franchises. For example, Gundam mobile suits are about 60 feet tall, pacific rim jaegers are 240-280 feet tall, and Warhammer 40,000 titans range from 40 foot tall ‘scout’ titans to the imperator class, which are at least 200 feet tall.
To make a ‘giant’ mecha, increase SM by 2, decrease Hnd by 1, double height, ST, HP, and DR, and multiply LWt. Load and Cost by 8.
A ‘colossal’ mecha increases SM by 4, reduces Hnd by 2, multiplies height, ST, HP, and DR by 5, and LWt. Load and Cost by 125.
A ‘titanic’ mech increases SM by 6, reduces Hnd by 3, multiplies height, ST, HP, and DR by 10, and LWt. Load and Cost by 1,000.
The mass of weapons carried can increase by the same factor as Load. A simple rule of thumb for scaling up weapons: multiply calibre, damage, range, and ST by the same factor used for the mecha’s height, ST, HP, and DR (e.g. a giant particle beam does 5d×40 (5) damage, has Range 12,000/36,000, and ST 200). Raise Acc by the amount the mecha’s Hnd was reduced (e.g. +1 for a giant weapon) and increase Bulk by the increase in the mecha’s SM (so giant weapons have Bulk −12). Weight and cost increase by the same factor as the mech’s.
Alternatively, a mecha can mount an array of smaller weapons instead of one big one. A giant mecha can mount 8 normal mecha weapons instead of one giant one. A colossal mecha can mount 16 giant-sized ones instead of one colossal one, and a titanic mecha can mount 8 colossal weapons instead of one titanic one. Such weapons are usually linked, so simply multiply RoF and shots by the number of weapons.
A giant mecha can have up to 4 crew without taking the Occupancy +1 feature, while a colossal one can have up to 25 crew and a titanic one up to 100. This number can be doubled by taking an Increased Occupancy feature (this is identical to Occupancy +1 except it adds more crew stations). Whether additional crew are actually required depends heavily on the setting.
Movement is also setting dependent. In some, even very big mecha move like humans, so multiply Move by the same factor as height, ST, etc. In others, larger mecha are slower and more ponderous: Move becomes 8/12 for giant mecha, 5/10 for colossal ones, and 3/9 for titanic ones. The Move ±5 feature becomes Move ±2/3 for giant mecha, Move ±2/4 for colossal mecha, and Move ±1/3 for titanic mecha. Jump jet movement is typically not adjusted for size.
Multiply the range of any sensors or communication systems by the same factor as the mecha’s height, ST, etc. The Acc bonus they grant is increased by the same amount as the mecha’s Hnd was reduced by.