If you've got Illustrator, you could load Jakobud's plans in EPS format. These are scaled at 1"=8", you could work with scales, but I find scales in Illustrator to be very confusing so it's probably best to work full size.
So to blow it up full size you'd select everything and multiply it by 8 (in the Transform palette, where it says W and H, put a *8 beside each dimension).
However, if you want it scaled to 3/4 of the original size, you'd select everything and multiply by 6.