tl;dr: I had a look at the website, and I have some understanding of how these things work. I believe that a minimal level of detail regarding the abuse is sufficient for the Commission's needs. If in doubt, the Commission has a 1800 number, and I'm sure they'd prefer for you to call them than for you to feel anxious.
Long version:
So, I had a look at the website and the terms of reference.
Terms of Reference, Child Abuse Royal Commission
It's worth noting that the instructions from the Queen to the Commissioners includes the following:
AND, without limiting the scope of your inquiry or the scope of any recommendations arising out of your inquiry that you may consider appropriate, We direct you, for the purposes of your inquiry and recommendations, to have regard to the following matters:
e. the experience of people directly or indirectly affected by child sexual abuse and related matters in institutional contexts, and the provision of opportunities for them to share their experiences in appropriate ways while recognising that many of them will be severely traumatised or will have special support needs;
Elsewhere on the site, I noticed that the law has been changed in order to assist the commission in meeting this requirement.
Also worth noting - the link on the website does not say "Make a submission" or "Submit evidence". It says "Share your story." Having made submissions to other Australian legal processes, I can assure you that this is a very big and important difference in wording, and it reflects a big and important difference in process.
Another thing is that all Australians have a legal right to silence - this doesn't just apply to defendants, it applies to accusers as well. So if you share your story and it is passed on to the police, it is possible that the police will want to ask you some questions. You cannot (legally) be forced to answer those questions.
How much detail should you provide? I think the answer is "How much detail do you want to provide?" The terms of reference tells you what the Commission has been instructed to investigate:
a. what institutions and governments should do to better protect children against child sexual abuse and related matters in institutional contexts in the future;
b. what institutions and governments should do to achieve best practice in encouraging the reporting of, and responding to reports or information about, allegations, incidents or risks of child sexual abuse and related matters in institutional contexts;
c. what should be done to eliminate or reduce impediments that currently exist for responding appropriately to child sexual abuse and related matters in institutional contexts, including addressing failures in, and impediments to, reporting, investigating and responding to allegations and incidents of abuse;
d. what institutions and governments should do to address, or alleviate the impact of, past and future child sexual abuse and related matters in institutional contexts, including, in particular, in ensuring justice for victims through the provision of redress by institutions, processes for referral for investigation and prosecution and support services.
So, if you're conflicted about whether they need a particular detail or not, then the question to consider is "Would that detail make a difference in terms of the investigation that the commission is doing?"
And, last but not least, I notice that the brochure in the "Tell your story" section has a 1800 number for enquiries. So, once you have a level of detail that you think might be enough, you could consider ringing that number and asking the question "Can I read you my description of what happened so that you can tell me if the Commission is likely to need more detail?"