» What is a good sample size to determine a meaningful ROI?

An accurate assessment depends on the size of the game.  A smaller game offers a reduced expected variance, which leads to a more rapid convergence to a meaningful ROI.

 For single-table SNG's, you should get a good feel after 400-500 games.  Big-time SNG grinders have said ROI really gets steady around 2000 games.

 For multi-table tournaments, especially those with huge fields, one could argue the sample size is bigger than the number of MTTs you could play.