GIMP Version: 2.6.11
Operating System: Windows
OS Version: XP - SP3
GIMP Experience: Beginner Level

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Hello everyone..i am a model car builder. and i was thinking about building a model car but wanted to make my own decals for this project..and i wanted to use gimp to make them with...So here is my question...how do i know how big the picture is gonna be once it is printed out before i print it out?..in other words how can i be sure that the decal will be the right size for my project before i print them?...the model is gonna be 1/25 scale...and the decal will be for the door of the truck i will be building..

Image>Print size

ok thanks

No problem.
If you need more precision:
Real door W/H in mm/25=x mm. Or scaled model door dimensions in mm.
Change Units in GIMP Preferences>Default image from px to mm.
Scale your door image to necessary size.
Use Measurement tool to verify door size (it shows mm too).
Print size> change from pt to mm.

That's what I usually do, but for the project like this one i'd probably use Inkscape.