What she can take depends on how tall she is and how narrow her pussy entrance is. I have 19cm thick cock and breaking in happens with thick cocks because the head has to be thick. What you need is girth to length ratio. The delirium only happens when she is psychologically ready. A good bull knows how to do that and it makes all the difference. Your actions as a cuck are important for her to enter into a delirium. If she gets properly aroused days in advance with psychological preparation she will be cumming just from me laying hands on her. Blindfolded eyes, surprise her with sex. Have sex, invite bull without her knowing, quietly swap with bull, she will lose her mind over unexpected penetration by the bull. First time has to be special. Condom removal during her squeels and her begging for me not to cum in her. There is plenty one can do to drive a woman mad for big cock. Depends on her libido too. On her personality. On her interests. Many factors are at play here and they have to be tuned like a guitar so when I break in she sounds like one. That is why finding a good bull is hard. Finding a big cock one is easy but sex without psychological connection is like eating sand or fucking a baseball bat.
The more sex with the bulls she has the less she will enjoy smaller cock but if she stops her pussy will readjust back to smaller size. So if you are 6,3 go with 7 to 8 first if you cannot have REGULAR bull. Don't be led on by size. If its too big 8-9 inches, it will hurt and will be the end to the kink. If it is thick and the bull is dominant; for the first time she will have a wild time. If you cannot have a bbc often, going for big will stretch her but then you will slowly stop interesting her sex wise. If you cannot provide dominant bull REGULARLY when she is stretched she will start losing interest in you and will get annoyed for many reasons.
The longer dicks are for extra psychological pleasure where the idea of not being able to fit in balls deep turns her on. If cock is thick her vagina will stretch sideways and will be shorter. So thick cock can be short but thin cock HAS to be longer. She would have the stretch and the appeal of not being able to go balls deep in either case. The other thing many ignore is hardness. Look for a hard cock. It can be big but if it is not hard she cannot ride it well. Bull would have to be able to lift her up and rail her against the walls, ceiling, tables, doors. You get me. Stiff big cock over floppy huge cock. She will appreciate details because bull has to be a bull. A raw dominant power that overhelms her and makes her feel small, safe and secure and in return she gives herself to his dominance.
The world is not ideal so people sacrifice certain preferences. In my experience, psychologically savvy, dominant bull generates more pleasure then otherwise standard BBC. Finding one is luxury but it is up to you to accept or refuse it.
For example, I am thick and so far only good feedback because I tune the couple and she cums in delirium from idea of being owned by a big cock rather than actually being owned. It is all in your head and thats what most of people in lifestyle seem to forget or are unaware of.