Players can 'scale' the list you've created to their liking; a large number of players (and the units in most of the books) place the equivalent of a modern MBT (M1A2/Challenger II) at Signature 3. Keep in mind that a vehicle's Signature is not just a reflection of physical size, but also includes the heat and electronic emission that the increased number of systems and weapons produce at lower Signature ratings.
The scaling I usually use when creating units is:
6: light recon/Jeep
5: recon/light tank/1-ton pickup
4: medium tank/ deuce-and-a-half
3: MBT
2-1: Heavy MBT
0 to -1: 'Light' Superheavy
-2 to -4: superheavy/heavy walker
-5 to -12 BOLO/Titan territory
This keeps gameplay running at a reasonable clip as the majority of units are eliminated with a single penetrating hit. There's also less record-keeping involved when your MBT don't each require their own damage tracking.
