The max cookie is size not explicitly defined. However, there is an implicit limit of sorts. Namely, when working in "HTTP" internal-protocol mode, vTM would always buffer complete set of headers, and that buffer is :
64KB for HTTP/1.x (unless changed at VS > * > Protocol Settings > Memory Limits > max_client_buffer);
256KB for HTTP/2 (unless changed at VS > * > Protocol Settings > HTTP/2-Specific Settings > http2!headers_size_limit).
Both figures is total size of all headers (including cookies).