A two group nonparametric effect size
Field (2005) suggests an effect size associated with performing nonparametric group tests. This is analogous to Cohen's d for parametric group testing. In particular on pages 531-2 of his book Field suggests computing a correlation:
correlation = z /sqrt(N)
where z is the z statistic that SPSS produces when you do the Mann-Whitney or Wilcoxon tests under analyze>nonparametrics and N is the size of the study.
This is a correlation so can be compared to the rules of thumb suggested by Cohen, namely, <0.1,0.1-0.3 and 0.3-0.5 for deciding if it has a small, medium or large magnitude.
Reference
Field A (2005) Discovering Statistics Using SPSS. Second Edition. Sage:London