As big a bra as you can wear and feel comfortable with. Some of the dressers on here have a condition where their male breast tissue has grown (edit: gynocomastia) and do require bras to hold everything in place. Others have sensitive nipples, so the material of the shirts they wear rubs the nipple tissue and causes discomfort. And of course, others wear bras for the thrill of wearing something pretty and feminine under their clothes.
If you look online on how to measure for a bra, you can figure out about the size you need. If you are daring enough, you can actually go to a ladies' store like Lane Bryant and be professionally measured and have someone make recommendations about what you would need.