How do I create a custom sized design template

I need to create a 20" x 30" "paper" (the size of foam board) to create aircraft models and measure them in inches.  The wings (1:1 scale) are 2 - 18" spans for a total of 36".  Fuselage 34.8"  (3 parts).  The wings will fold over with the bottom being 4" wide, the top slightly over 5" - 4" wing -1" aileron and the leading edge curve with a 1/4" thick spar to help form them.  Therfore, each wing drawing will be 18" X about 9-1/2".  I will then have to print 8 pages per 20 x 30 sheet (2 - 11" pages each for the 20" height and 4 - 8-1/2" pages for the 30" width).  Also, is it possible to draw a rectangle by putting in coordinates e.g. x = 4"; y = 15". Any help would be greatly appreciated.  

I hope what follows is sufficiently understandable and I apologise if it covers things you already know.
A good start for your custom template is
- Create a new blank document
- From the Options menu choose Application Preferences and in the Paths tab type in the path to where your templates will be stored.
If you will consistently be working in Inches then in the Application Preferences/Defaults tab change the drawing units to inches.
- Again from the Options menu choose Current Drawing Preferences
- - In the Units tab choose Inch as the unit if it is not already set
    (This means that, when the Grid Status on the Status bar shows 1/10
      each small division will represent 1" while each larger square will represent 10")
    Given you wish to use a 'canvas' of 20" x 30" most of your drawing will be done with a Grid status of
    0.1/1.0, 1/10 or 10/100 to see your various components, depending on their size.
- -In the Paper tab select Letter (11" x 8.5") for the paper stock you will use to print.
- -In the Dimensions tab leave the General Scale at (or change it to) 1.

If you wish the rectangle to be a permanent part of your template then draw it first (as described below) before saving the named template.
When your template is ready do a Save As giving the document a suitable name e.g. Aircraft Template.dxf and save to the place you chose for templates in the Paths tab (above).

- Because the units of measurement are now inches to produce the rectangle I suggest start to draw it from the absolute zero point 0,0 (marked by a red cross) as the first corner and 4,15 as the second corner.

There are principally two ways to do this:
 1. On the Command Line type rect [Enter] (i.e. use the Enter key) the Command Line window above the Command Line will show Command: rect (rectangle) while the Command Line itself prompts "Specify first corner". Type 0,0 [Enter] and the prompt changes to "Specify second corner", at which type 4,15 [Enter] and the rectangle will be drawn; or
 2. Using the mouse first turn on Snap to Grid then choose Rectangle from the Tools menu/Line submenu and left click on the absolute zero (red cross). After releasing the mouse button move the mouse pointer to the coordinates 4,15, noticing the rectangle expand as you do so, then left click again to complete the rectangle. Now right click to cancel the use of the rectangle tool.

When you come to printing your pieces you will want to overlap a little to affix them to your foam board.
Choose Print Preview (not Print) and then choose 1:1 at the top of the range of scales that appears when you click on the down arrow next to the current scale (which will almost certainly not be 1:1 to begin with).
Now an item 6" in length will be printed at the correct size on your chosen (11" x 8.5") stock.
Change the zoom ratio either up or down within the Grid Status 1/10 until you can see the complete representation of the printed page. If you don't see the margins the zoom factor is zoomed in too far.
When you can see the paper margins, place the mouse pointer inside and hold the left mouse button down whereupon it becomes a hand allowing you to drag the "paper" around underneath your drawing items. In this way you can choose which items will print when (from within Print Preview you can choose the Print option). Choosing a different area each time you "Print Preview" will allow you to progressively print the whole drawing.