This is how I resize drawings..works great!

but you need a good photo program to do it (something that can measure,in inches or Centimeters or some real unit.)
It sounds complicated!
but its dead-easy once you get it down..
its much more complicated to explain it that it is to actually do it!
Scan the original drawing..doesnt matter what size or scale it is..
scan it at a good ppi, like 200 or 300 ppi.
Bring the scan into your program (I use photoshop)
measure the entire width of the scan, from edge to edge..
lets say its a HO scale drawing of a locomotive and the original drawing is on a 8x10 sheet of paper..
so the total width of the scan is 10 inches. (completely ignoring the drawing at this point..
you only care about the total width of the piece of paper the drawing is on)
Now take one known prototype dimension..lets say the wheelbase of the prototype engine is 60 feet.
60 feet X 12 = 720"
measure the actual distance on the drawing for the loco wheelbase..if its HO scale it would be 8.27"
Now you take the scale you want to convert the drawing to..lets say 1/29 scale.
720 prototype inches divided by 29 = 24.82" - thats how long the loco wheelbase should be in 1/29 scale.
So, you have a drawing where the length of the wheelbase is 8.27" (HO scale)
you want to make a drawing where the wheelbase is 24.82" (1/29 scale)
using those two dimensions, plus the overall width of the paper (10 inches) the formula is simple:
10 X
----- = -----
8.27 24.82
Solve for X
10 X 24.82 = 248.2
248.2 divided by 8.27 = 30.01
X=30
Resize the whole drawing to 30" wide.
done!
print out at 100%
you know have a drawing of the loco in 1/29 scale..
This works for any size original drawing, and converting to any scale..
you only need three numbers, and solve for the fourth..
the fourth is the length of the final drawing.
Scot