Dessignjet 750C plug

Hewlett Packard Dessignjet 750C plug. When I try to print an 8 1/2 by 11 and I
tell it to enlarge it to an 18 X 24 the printer prints the paper larger but
the image is still an 8 1/2 X 11. The goal is to to print the image bigger
not the paper size. Can you help me.
WINDOWS NT4 AND printer FORM MICROSTATION 95 OR MS WORD DOCUMENTS
Please send details on how to do it.