Okay...I made two pieces of knitting - one in garter stitch, the other in stockinette.  I took before and after photos...

This first piece started out (cast on 30, knit for 20 rows) 11 inches by 4 inches and ended up stretched width-wise, but only 3 inches tall.

This second piece (stockinette, cast on 20, work for 20 rows) started out 6 inches wide and 6 inches tall.  It ended up 7 inches wide and only 4 inches tall.

Lessons learned:

• bind off very (VERY) loosely - in fact, try knitting the last row and binding off with a huge pair of needles.
• get to the washed piece quickly in order to block it while it's wet (although this may not be as important if you bind off loosely)
• stockinette and garter stitches don't act the same.