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

Answer: 
[Stitching lengths:](http://www.the-sewing-partner.com/sewing-stitch-adjustments.html)

> 
> 2mm is the short stitch length that should be used for lightweight
>  fabrics, for satin stitching, and decorative stitching. 
> 
> 
> 2.5 - 3mm is the average stitch length range that should be used for medium weight fabrics. 
> 
> 
> 4 - 5mm is the long stitch length range that should be used for
>  basting and topstitching.
> 
> 
> 

[Stitches for Dummies puts it this way:](http://www.dummies.com/how-to/content/making-machine-stitches-work-for-you.html)

> 
> * The average stitch length for mid-weight fabrics is 2.5 to 3 mm/10 to 12 spi.
> * The average stitch length for fine fabrics is 2 mm/13 to 20 spi.
> * For heavier fabrics, basting, or topstitching, use 4 to 5 mm/5 to 6 spi.
> 
> 
> 

Personally, I baste by hand and use a shorter length stitch for strength when necessary. This [video](http://www.howcast.com/videos/454064-how-to-select-stitch-length-sewing-lessons/) explains where I am coming from!
> 
> 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.
> 
> 
> 

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.