Im having a hard time understanding sizes with this plugin.
This is from the plugin page:

radius - Number amount - Set the radius of the zone (if using a sphere shaped zone)
size - "width height length" - Set the size of the zone (for use when creating rectangle zones. Be sure to type the width, height and length as numbers representing the size you wish the zone to be

How much is "1 amount" radius? 
And how much is "1 width" and "1 height" ?

We are working on creating an hotel, and i cant really figure out how big my zoning actually is.
It says "30" ingame, but i dont know how big "30" is..

Anyone that can help explain this?

Whatever size you choose should be indicated by a wire sphere that appears when you add a zone. You may need to fly up in the air and look down on it to see it,  as it might be outside the building and out your line of sight.  That's using the /zone radius command.  If you want a square zone you use the /zone size command.  I recommend sticking with the radius command because it's much simpler to use. 

I have found changing the size of square zones by doing it like this.
example: /zone size "x y z" "x-width y-height z-length"
/zone size "40 20 40"       change to what ever you need it to be.
/zone rotation 50             change to what ever you need it to be.