I'm thinking of using a brain wallet for btc storage. Yes i know all the talk about it being insecure. But what if I used a bunch of information that only I would know making a >100 character brain wallet ? What is the max length of a brain wallet?
A 256 bit hash of any string will result in a 256 bit (AKA 32 character) hash. For this reason, any string longer than 32 characters will collide with a hash of some other string, potentially one that is only 3 or 4 characters long. The risk is ridiculously low, of course, but it's still there.
Choosing a >100 character string to hash (which can certainly be done) is not necessarily more secure than choosing a 32 character string to hash.
Be super careful what you use as a password/passphrase, though. No full words or names, in any language. Do not use common phrases or quotes, or pieces of such. For example, using the first letters of all of the words of the Gettysburg address, although making for a long passphrase, is actually pretty likely to be stolen.
What you could do, however, is start with a root passphrase, hash it, and combine that hash with another passphrase, and repeat the process until you are satisfied with the result.