If any wind, the smallest that will do the job. In no wind, I like a 3x6' that has angled legs with sand bags. No assistant needed. I have a cross piece on it so can position it with one hand. It is always on the shadow side in studio. A silver white cover is perfect but also have a black white for subtraction or in studio, to eliminated light bouncing around. If minimal wind, the 40" held by an assistant or passerby. More wind, I like the trigrips have less wind resistance. If no wind, I have a 6.5" with b/w, silver/white and translucent and can be hung on 2 stands to soften or eliminate light. In mid day, if I don't want to hand the 6 footer, I have a translucent umbrella that can be held over subjects head to eliminate racoon eye and the assistant can hold a trigrip in the other hand as a main.