What is the maximum allowed limit on the length of a process name?

According to man 2 prctl:

PR_SET_NAME (since Linux 2.6.9)

Set the name of the calling thread, using the value in the location pointed to by (char *) arg2. The name can be up to 16 bytes long, and should be null-terminated if it contains fewer bytes.

So I'd go for a 16 bytes long buffer.

EDIT:

Let me back this up a little more.

Each process in Linux corresponds to a struct task_struct in the kernel, which is defined in include/linux/sched.h.

In this definition, there's a field char comm[TASK_COMM_LEN], which according to the comment refers to the executable name excluding the path:

    char comm[TASK_COMM_LEN]; /* executable name excluding path                                 - access with [gs]et_task_comm (which lock                                   it with task_lock())                                 - initialized normally by setup_new_exec */

Its size, TASK_COMM_LEN, is defined above in the same header file, here, to be 16 bytes:

/* Task command name length */#define TASK_COMM_LEN 16

Furthermore, quoting LDD3 page 22:

...

the following statement prints the process ID and the command name of the current process by accessing certain fields in struct task_struct :

printk(KERN_INFO "The process is \"%s\" (pid %i)\n",        current->comm, current->pid);

The command name stored in current->comm is the base name of the program file (trimmed to 15 characters if need be) that is being executed by the current process.