From my experience, size, not too small. One thing I noticed as sometimes I glue small, I mean really small pieces onto plugs that are more or less debris after fragging, and if I put them in normal strength lighting, they always bleach and die. If I put in really low light and good flow for a week, the heal. Give that a try, lower the light at first. I also mostly try to frag 1" by 1" at a min.