In this world, there is a way of categorizing Mecha sizes.

For example, Size 3 Mechas mentioned in Chapter 2 are used for ones between 5 to 8 meters tall. The chart goes like this:

Size 3: 5 to 8. Size 4: 8 to 12. Size 5: 12 to 15.

But after Size 5, things change, and we enter big ones.

Size 6: 30 to 35. Size 7: 45 to 60. Size 8: 70 to 100. And finally, the tallest one is Size 9: 154 meters tall, which Earth only has one.

Any Mecha with unfitting length is called by the closest shorter size, but an + and how taller the Mecha is from the size is added. 

For example, an 19 meters tall Mecha is called Size 5 +4, or a 61 meters tall Mecha is called Size 7 + 1

Now, Sizes 1 and 2. They don't indicate length but rather weight.

For example, a Size 4 Mecha lighter compared to its peers is called 4-1, while one heavier than is called 4-2. No 1 or 2 means Mecha is in the standard weight class.