Given that limited resources are a constraint for small and medium enterprises to design and adopt corporate social responsibility, perhaps a group effort by the enterprises may prove more effective.
...more effective, very probable. Small and local positions use to be more effective for constructing believable strategies with local (positives) consecuences. Even from the point of the real values of RSC (versus the big RSC Reports than remains only on the paper, or in the web.
I think Corporate Social Responsibility activities can probably be more effective in small and medium enterprises if designed and executed by effective management and if enough financial resources are allocated for these activities.
Stakeholder Engagement may help SMEs become more effective as they possibly have less scarce resource when compared to their larger counterparts. (You can refer to the Resource Based View Theory - RBV)! If you would like to research further , I suggest Orlitzky's publications!
I think it really depends on the strategic positioning of a company (differentiated by the industry sector they are in). Some sustainability initiatives may be more material to a group of companies compared to others. The real question in this case is would small and medium enterprises cooperate in CSR initiatives?
I have not come across with many such parameters so far. One will have to design depending on the objective of the study.
From the point of view of the corporate sector, Resources/ amount of money ( may be measured by percentage of profit spent)
From the point of view of other stakeholders, channel of implementation
From the point of view of the beneficiaries, the impact of the activity which may be measured in terms increase in income, improvement in health (can be measured by expenditure on health services) etc.