The size of physical spaces - especially in Western countries - is most often dictated by licensing agencies.  The specifics depend on the age of the children in the space; the regulations also govern the number of children allowed in the space at one time. 

In non-Western countries, there are generally not so many regulations on the size of the space nor on the number of children.

Maria Montessori said that for the Casa the best number of children is between 35 and 45 children, between the ages of 2.5 and 6+ years.  There was to be one trained teacher and an assistant.  This number today is difficult to achieve in Western countries - though experience has shown that whenever possible, there is a considerable difference in what happens in the environment. With too many children for one trained teacher to “teach”, the children take over.  They help each other, they give lessons to one another on how to manipulate various materials.  The environment becomes the “Children’s House” – not an environment completely controlled by an adult.

It must also be remembered that in more traditional settings for children, there are toy boxes and bins, eliminating the amount of space needed for a Montessori environment as our environments are ordered and organised with every piece of material - of which there are many - visible and available for children at any time. The training of a Montessori guide covers not just materials to be presented, but more importantly, human development for a minimum of a 3-year age span.  

When given an option, choose a larger space rather than a smaller one.  If the space is too small, either the material needs to be pared down - not all materials available at any given time - OR the number of children needs to be reduced.  Neither of these options is desirable. Children in a Casa are still refining their movement skills - which sometimes requires a bit of space.  If the space is too crowded, there are many more instances of conflict. 
 
To directly answer the question: someone needs to know the local regulations, measure the available space and make a determination.  Having too few children does not make for an optimal Montessori environment, and too narrow an age range also works against the pedagogy.