Click the picture for a larger size.
The sign I made for my car (truck) was: 25 inches wide X 10 inches high, on the outside dimension. I had also asked for rounded corners.
I started by making a drawing using OpenOffice ("Draw" .odg): Drawing
The drawing was made 1/3 the size of my intended final printed size so it could be printed on letter size paper (8.5" x 11"), if needed.
Next I made a PDF file, using OpenOffice, and displayed it on my computer screen as big as I could get, which was about 12" for the printed portion. I then walked back, away from the computer, to where it became harder to read and that was about 4 feet. I now knew that the sign at a 12.5" width could be read at about 4 feet. I asked the printer to enlarge my sign to 25" for a viewing distance of about 8 feet.
The viewing distance gets longer as the sign gets bigger. For example to make the viewing distance 20 feet, 5 times further than the computer test distance, enlarge the finished sign by 5 times. This would be approximately 1 foot times 5 = 5 feet wide and about 3.5 feet high, in my case. This size of a sign would be to large to fit on my truck. To shrink the size and keep the viewing distance, I would need to maintain the printed character size, and redo the message using less characters.
Things to think about:
A) Since the magnetic signs need a relatively flat steel surface, the intended surface needs to be measured before deciding on the sign's final dimensions.
B) It may be that 1 sign or more are needed, but the cost will usually be lower per sign if more copies are done at the same time. Asking the 1 each and multiple cost can be helpful. Also, one color may be less than two since each color may need to be printed separately.
C) The store I bought my signs at was:
D J Electronic Graphics,
Sunnyvale, CA USA