I used my CAD program to come up with these dimensions.
They should be as accurate as you can get.

On the chart below, "a line this long" represents the blue arrow in the yellow area. "A template this wide" represents from the red line to red line, which is just doubled in the third column to give the width of the finished star.

WARNING: You are going to loose a little (about 1/2") in the finished size of the star because of the seam allowances. 

If this is a great concern and your star *must* be an exact size (and to confuse you even more!) you can make your diamonds a little larger and then trim them down to an exact size after you sew on the strips.

Okay, here is the deal with the size of your point and how big a quilt it will make. 

See the dark yellow area? You will see that the length of the diamond from tip to tip how you will draw the line (where the arrow is) doesn't make the width of the quilt when you double it because the diamonds actually sit on their side.

To figure how wide your star will be from side to side, which will actually be the width of your quilt, you need to know the distance your star will be from the red line to the red line as the diamond sits on it's side. I used a CAD program that will figure dimensions to make this chart.

I haven't tested each size, so first make two paper templates, lay two out on their side like the yellow diamonds in the diagram. Lay them against something straight (edge of table or line on tile or wood floor) to make sure they are tilted right, and measure to make sure the size is what you want. If it's too large, just trim off an equal amount off each side of one end of the diamond and re-measure until right . If too small, you will need to lengthen your line and make new diamonds.

Read Across left to right---->

A line this long
Will make a template this wide (red line to red line)
For a star this wide
4 1/4 inches
4 inches
8 inches
6 1/2 inches
6 inches
12 inches
8 3/4 inches
8 inches
16 inches
10 3/4inches
10 inches
20 inches
13 inches
12 inches
24 inches
15 1/4 inches
14 inches
28 inches
17 1/4 inches
16 inches
32 inches
19 1/4 inches
18 inches
36 inches
21 3/4 inches
20 inches
40 inches
23 3/4 inches
22 inches
44 inches
24 inches
48 inches
28 1/4 inches
26 inches
52 inches
30 1/2 inches
28 inches
56 inches
32 1/2 inches
30 inches
60 inches

Chart of standard mattress and quilt sizes.

This chart will give you a general center star size, plus a guide for figuring the additional strip size you will need at the top and bottom of the star to make it a rectangle. On the quilt at the left, you add a strip of fabric to the top and bottom of the star (Center panel size below) to make it a rectangle, then add the outside border which is one fabric piece to make the 12 inch side drop for the quilt.
On the right quilt sample, the strips around the star are two different fabrics. A narrow strip is added to each side of the star first and then the top and bottom strips are added. That brings the star more in the center of the quilt. Then you can add another border to make up for the 12 inch drop on this quilt. This option makes the star "float" on the quilt top.

Now please note: you can add a larger drop than 12 inches is you have an extra thick mattress or like your quilt longer.

...or...you could be really ambitious and piece a border. 

This border is set on point and I can use my quilt design program to figure out the dimensions for the pieced blocks and the setting triangles.

If you don't have quilt design software, you can use the formula for figuring your setting triangles in step 7 to figure the setting triangles in the pieced border too.

There are many pieced borders you can use on a quilt like this.
A woven ribbon border is very lovely and something that looks "Navajo" will give it a great "Southwest" feel.

Standard Quilt Sizes

Twin quilt Size: 64" x 99" (based on standard twin mattress size of 39" x 75" with a 12" drop") 

Twin Center Panel: 40" star plus top and bottom strip to measure 75", then add drop.

Full quilt: 79" x 99" (based on standard full/double mattress size of 54" x 75"and 12" drop on all four sides)

Center Panel: 54" star plus top and bottom strip to measure 75", then add drop.

Queen quilt: 84" x 104" (based on standard queen mattress size of 60" x 80" and 12" drop on all four sides) 

Center Panel: 60" star plus top and bottom strip to measure 80", then add drop

King Quilt: 96" x 108" (based on standard king mattress size of 72" x 84" and 12" drop on all four sides) 

Center Panel: 72" star plus top and bottom strip to measure 84", then add drop.

Crib Quilt: Varies...

Center Panel: 27" star plus top and bottom strip to measure 50", then add drop.

You may not want 12" drops on a crib quilt, so adjust borders to your liking.

Cot or sofa cuddle quilt: 54" x 99" based on cot mattress size of 30" x 75" and 12" drop on all four sides)

Center panel: 30" star plus top and bottom strip to measure 75", then add drop.

Here again, you may not want a 12 inch drop on this quilt, so adjust borders to your liking. 

Now that you know what size star and quilt you want to make, go to next step: Fabric choices
Main Introduction Page 2) How big a star 3) Fabric choices 4)You need a 45 degree diamond 
4a) If you have a 45 degree diamond
4b) draft using a protractor.
4c) draft  using a compass.
4d)  .pdf format printout
5) Making a star point in any length
6)  Sew 
    Page 1
    Page 2

7) figure size of background squares and triangles

8) Sewing together star points/ setting in the background
9) Adding Borders

9a) floating blocks

10) *really easy* Lone Star quilt.