The first question for you to think about is how many people do you think the shelter should be able to handle at one time.
Here, the answer works out to be that one should allow 10 sq. ft., maybe a little less, per person, so for a few popular sizes this means:
This formula also works well for large transit transfer shelters.
The second question is a real estate question about the width, length or orientation of the real estate available to work with, and finally the location and size of the shelter opening for access.