Basically you should use a vector export format like DWG or PDF. You won't get textures or shadows, though. The basic rules:
http://www.sketchucation.com/forums/scf/viewtopic.php?f=18&t=9281

For raster exports it is different. Pixels do not have a scale. You can either print the images directly to paper or PDF using a PDF printer (still using a standard view, with Parallel Projection on), or do something in this line:

Draw an empty rectangle around your model with dimensions you know. export the plan view with parallel projection to an image. open the image in photoshop trim away everything outside your "frame". Photoshop can do this automatically if you have an uniform background. now you can resize the image to the dimensions the frame should have, with or without resampling. if you need to you can now crop away the frame. Photoshop remembers your DPI setting so the image should remain at the correct scale. when printing from any application, remember to turn off all page scaling in the application or your print driver. For instance, Adobe Reader and Adobe Acrobat default to fitting the page to available printing area, which produces extra margins and a slightly downsized image

Anssi