void PlotLength (length) BPLT.C Level 1
float length The long dimension of your plot in inches (cm). This number should match the specification for the plot length you gave when you set up your printer for use with GraphiC if you want your specified dimensions to correspond to printed output. The default length is 9 inches.
Because GraphiC supports variable output page sizes, GraphiC needs to know how to map the internal pixel coordinate system to your output device. You are free to specify this scaling, but we recommend that 
it match the plot length of an L printer dump that you specified when you ran EQUIP (in DOS) or set your printer defaults (GUI environments). If you make any other choice, the sizes on your printer output will not match the specifications in your program. You may call PixPerUnit() instead of PlotLength() to specify the scaling in terms of pixels per inch.

This function must be called before page() or the scaling will not be correct.