Why do you want a fixed work unit size for each thread? Given the vagaries of multitasking, some thread is sure to finish a fixed length task before another doing the same size task. Why not just pour all your data into a Thread::Queue and have a pool of workers servicing this queue? If you have a very large amount of data it can also be handy to have a size limited queue, BrowserUK gives a great example here: Re^5: dynamic number of threads based on CPU utilization.

Cheers,
R.