[ASM] : "Request length exceeds defined buffer size " - How to increase the limit ?

I am not aware of an option that does this for a specific URL.
My suggestion is to change the long_request_buffer_size as described in K01235989 and activate an iRule/LTM policy that blocks larger requests on all other URL and virtual servers (if needed).

Alternatively you can bypass the ASM for the path completely (K22021244)