In my interpretation, the distance between two parallel walls is 30cm. This allows for different wall thicknesses depending on local suppliers. Also, it is important to note that 30cm x 30cm "tiles" need to be placed within the maze. If the inside distance is less than 30cm the tiles would not drop into the maze.
For example, the rules state:
1.2.4 A tile is defined as a 30 x 30 cm space, which is aligned to the grid made up by the walls.
The rules can be revised to make this clearer.
In addition, the "foyer" is simply a space that is greater than one tile wide.
1.3.2 Paths will be approximately 30 cm wide and may open into foyers wider than the path.
RoboCup International 2018 Montreal Canada LOC