It should do that, or I'm misunderstanding your question. Are you actually changing the model size? or, are you trying to fudge scaled dimesions? are you using viewports, or drafting pallet? Are your dimensions associative?
TC18 / 38.5 Platinum, AL5. XP, Core2 Duo E6750 OC'd 3.0Ghz, Render test: 3mins 3sec.