if i was you, I will pick one important elements of all the research you've done and expand on it. I will probably pick a research experience that fit into the university goals or at least, the research you learn the most from. Cos from the way you described it, if you decide to write everything you have done research wise, you would go over the words limit or it will be more like a "list" form as opposed to nice coherent paragraph form. So I prefer you pick one or two, expand on it and talk about what you learn during the research, and how that can help you to become a better physician. Do u think that experience fit in the university goals, etc? if it does, it would not hurt to include that as well.