The easiest way to make a poster is to do it on an ordinary, letter-sized page and then have a printer blow it up. To do this you must use vector graphics, not bit-maps. Vector graphics can be blown up without loss of quality, bit-maps cannot. If you need to insert a photograph or some other bit-map, make sure that it will have at least 300 pixels per inch at the size you want to show it. (E.g., a photo you want to show at 5" x 7" should be at least 1500 pixels x 2100 pixels.)
When you finish laying out the poster, you will have made a computer file created using non-standardized algorithms and non-standardized fonts. Not only are these algorithms and fonts not standardized across systems, they are likely to vary with different versions of the same system. Thus, if you transfer that file to another computer for printing, you are likely to see surprises. To avoid surprises, save the poster as a PDF file. In the Print menu, click on the PDF pop-up menu then choose Save As PDF.
You can now send the file to any printing service (or to Gary Weatherill) and see it enlarged exactly as specified—but don't send it yet. When a small object is enlarged, it tends to look like a small object that has been enlarged. The perception of size constancy does not exist solely in the lab: to look right, large objects usually need to be scaled differently from small ones. Since you have been working on a small-scale mockup, proof it at full size expecting to make changes. However, poster-sized printouts do not come cheap, so make the proof on ordinary paper, with the enlarged image covering multiple sheets. To do this, open the PDF file in PStill and choose "Tile Poster" under the "Conversion" menu.
Finally, if the printout is to contain colour photographs, make sure that you have calibrated your monitor appropriately.