Hi ss02150
maybe you can have a quick check of the domain size and see what the difference in result is, if there is no difference I would say you can stick to the smaller fluid domain.
For the initial condition, you can just set a mean value, so I would go with a good average value of your ABL (free stream velocity) .
In regards to the residuals, it depends on the results you are interested in. If the residuals for example high further away from the building, it won’t have an influence on your result. Therefore I would focus on the results you are interested in.
Best regards
Sebastian