From what I’ve read, 9 inches seems to be that point where a good portion of women cannot handle what you’re packin’. This is BPEL too. I know that there are many women who can handle much more, but in most cases, once you get 9 inches into her, you can start causing some damage. However, this is just length. You asked size in general. You can have a 6 inch long dick that’s 9 inches thick (as ridiculous as that may sound) and most girls couldn’t handle that either. Again, my own input from what I’ve read seems to be that girls can’t handle much more that 25 cubic inches. Some girls can take that plus much more, but you asked for a line, and that’s where I’d personally put it.