Mobile Frame Zero: Rapid Attack can be played with most any size of frame, the common format is to play on what is known as a 7 plate scale. LEGO is built on a set of geometric relation between pieces, 3 plates equal a brick in height and so on. By choosing a scale, you help players make frames of similar sized for measuring and gameplay.
In 7 plate scale, the average human is as tall as 7 LEGO plates, or two bricks and a plate.
In 6 plate scale, the average human is as tall as 6 LEGO plates, or two bricks. This scale is also the size of Lego's microfigures, the same ones seen in most LEGO board games.