If the canvas size says 30x15 then that is the size it will print at. Keep in mind that many printers don't print all the way to the exact edge of the paper.
You can also do the following, if you feel your eyes are tricking you:
With the document open, press Ctrl+R (for rulers). This will show you the rulers along the top and bottom. If you double-click on the rulers, you can adjust the units of measure.
And just as a tip, I like to press F after I open a document, to toggle throught the different view options. It's just a nicer use of the space you have on screen while working on an image.
Ok, and now resolution. That plays a big role and it is worth spending a few minutes learning the basics of resolution. It will make life easier later when you sometimes wonder why things don't look like you thought they would.
Resolution doesn't change your canvas size, but it affects how things will look on that canvas. Depending on what you are doing, I would go with a resolution of at least 180 dpi for your canvas (and probably more like 240 dpi). Keep in mind that anything (for example pictures) that gets brought into this canvas as a new layer, will look bigger or smaller depending on its particular resolution. I made this little example to show you, because it's the kind of stuff that is better understood if seen than read.
The blue is your canvas and the black squares are "pretend pictures" of certain size and resolution. You can of course scale and enlarge those black squares on the canvas, but if you enlarge the little one to be as big as the bigger one, then you would lose detail and clarity of that photo (although in many cases it is hardly or slightly noticeable). Just play with it to see what I mean.
Notice that both "pretend pictures" would print out to be 10x10 inches if printed out as individual pictures. But the moment they are placed on this canvas, their difference in resolution (in relation to each other and to the canvas) causes them to "fit" differently on the canvas. Notice also that the bigger picture has the exact same resolution as the canvas. Therefore, that picture will print out as its true size - i.e. 10 inches, while the other one would print out to be 5 inches long because its resolution is half the size of the canvas resolution.
The final aspect is to know at what resolution you will be printing and make sure that the paper or media you will print to supports that dpi. Will you print at home or will you take it to be printed professionally? Knowing the printer resolution can help you decide what resolution to set for your canvas. Don't assume that those two resolutions need to be the same.
Read this about image and printer resolutions.
http://graphicssoft.about.com/od/aboutgraphics/l/blscanning3.htm
So, this is probably more than you expected, but I hope it helps. Now go at it and have fun.
