Question: When looking to sew a piece of fabric, what factors are looked at to determine the correct stitch length?

Answer: 
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> The theory behind stitch length is pretty straight forward. The shorter the stitches, the more will be packed into each inch of stitching, creating a tighter seam. The longer the stitches, the fewer within each inch, therefore, the looser the seam. Short equals tight; long equals loose. Another way to think about it is: short equals stronger and permanent, long equals weaker and temporary.
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I found [this article](http://www.sew4home.com/tips-resources/sewing-tips-tricks/setting-stitch-length-quick-look-how-and-why) very helpful in understanding the basics of choosing stitch length.
In addition to the good answers by other responders, I want to add that with a short stitch, you also increase the possibility of puckering, sometimes a longer stitch gives a much smoother appearance. 

When the stitch length is very short, yes, you can achieve some seam strength, but if the fabric is light-weight and/or has some stretch and the seam has none (short stitch length used), the fabric can tear away from the seam, or the thread in the seam can even break if pulled hard enough. 

There are so many variables with the fabric you are using, its weight, density, stretchiness, etc., that it's always good practice to do a quick test on a small piece of your fabric (including linings and interfacings if used).

Most newer machines have very good User's Guides with a lot of useful information, including tables showing best stitch lengths for different types of fabrics, best needle size, what foot to use to achieve a certain stitch, etc.