Here's the author's summary of the moderators of effect size:

perhaps more important, a number of moderators of experimental disclosure were identified with a random effects approach; effect sizes tended to be larger when studies included only participants with physical health problems, included only participants with a history of trauma or stressors, did not draw from a college student sample, had participants disclose at home, had participants disclose in a private setting, had more male participants, had fewer participants, paid the participants, had follow-up periods of less than 1 month, had at least three disclosure sessions, had disclosure sessions that lasted at least 15 min, had participants who wrote about more recent events, instructed participants to discuss previously undisclosed topics, gave participants directed questions or specific examples of what to disclose, gave participants instructions regarding whether they should switch topics, and did not collect the products of disclosure. Conversely, a number of variables that were originally hypothesized to moderate experimental disclosure were not significantly related to effect size: psychological health selection criteria, participant age, participant ethnicity, participant education level, warning participants in advance that they might disclose traumatic events, spacing of disclosure sessions, valence of disclosure topic, focus of disclosure instructions, time reference of disclosure instructions, and mode of disclosure(hand writing, typing, or talking). (p. 851)

The overall weighted effect size was .063 (p. 834)