I've had some very unsatisfactory results in 45 cal round balls. That being said if I had to use them or wanted to use them I'd go at least 50, and if it were my choice I'd use 54 or 58. I don't think you can get too big with round balls.
Of course shot placement is key, but even with good placement on hogs and whitetails my 45 was very dissapointing. Switch to maxi balls and a whole nuther story.
Jeff