How do you choose a stitch length when straight stitching an arbitrary fabric?
Stitching lengths:


  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:


  
  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 explains where I am coming from!