can I change the maximum lenght for a fixed passcode in AM 8.4 and AM 8.5 ?
I don't think so Ken, I cannot find the option anywhere in AM 8.4. Even if you assume that a Token should not have a PIN greater than 8 digits because the Token can only display a maximum of 8 digits, you can have a separate Fixed Passcode setting in the Token Policy, therefore it could work for a Fixed Passcode.
I checked, other customers have asked this same question and the response was 8 character max.
I think this could be an RFE, Request for Enhancement. However Product Management might decide this would make fixed passcodes too much like complex passwords, which could be seen as an undermining of the two-factor authentication strategy of Authentication Manager. But we could ask. Especially in light of so many automated testing and scanning applications that use fixed passcodes.
There is an RSA idea (RFE) for a 16 character fixed passcode.
But based on your hat icon being present there, it looks like you already voted for it. Tell everyone else at your company to vote it up too, and then we can bug the TAM to get some traction on this.