You're right - I thought that would set the global units as well (never tested it ) It definetly would make more sense if it did.
The only way round it I can think on would be to type in =(realsize)*(scale) for example to specify the far corner for an A4 sheet of paper when drawing at 1:50 you could type in "=297*50" for the x and "=210*50" for the y.
("=" tells it to evaluate the next bit as per an arithmaric formulae; you can use ()+-*/ and ^. It will just take the first number typed in if you omit the "=" at the start)
How do you change them for an individual entity then?