I usually start with medium and resize the modeling box to whatever I happen to need.  I usually try to make sure to keep all elements in the modules to scale.  I never resize an object in the assembly room since this makes it almost impossible to keep the scale consistent.  The trickiest things to get to the correct scale are angled pieces as I am sure you can imagine.  I really get annoyed by the scales available in Carrara sometimes.  So whatever scale I am using I try to keep at a base 10 inch, ft, etc reference.  Just remember to use the modeling room to resize and / or modify objects.  Doing it in the assembly room messes everything up.  Also try to use as few master objects as possible.  Use duplicate instead of copy and paste.  I try to picture in my head what I want to something to look like before I even start modeling it.  Then I mentally break it down into the fewest workable components and model each one.  Fpr archiotecture, the spline room is perfect and much easier than the vertex room.  And if you need to do something to an object in the vertex room later on, it is very easy to convert it.