how do you determine size and space for your levelsl?

Do you use, metric or imperial and what ratio do you use. for something like an FPS for example.
same size as irl?
is there a bkm tutorial for this or advice?

what is irl and bkm?

It is always a good idea to determine the references before you start modelling (references = where is the origin, what are the dimensions, how is the alignment?).

In general you can define the scale as you like. The numbers are decimal (I do not know if a different system is possible in Blender, at least it will not appear in the BGE). The size of a unit is often referred as BU = Blender Unit which is naturally 1.0.

The scale matters when you use the build-in Physics engine (Bullet). It assumes 1 BU is 1 metric meter. This means the default cube has a size of 2m x 2m x 2m. A character with typical humans size should be able to stand inside of this cube. If a character jumps from the cube it is a 2m fall. That can hurt (if it would be real live ;))

@Monster - IRL = In Real Life, I imagine.
@pdxDaniela - I would follow Monster’s advice that basically, you should build it based on 2 BU = 2 meters / 6 feet / roughly the height of a normal adult human male.