I want to increase the size of my individual sperm cells to be as large as possible. I have no ulterior motive here -- I don't think it will make me more reproductively successful or anything like that -- it's more of a body-modification thing.

On the theoretical side: The largest known animal cell is the ostrich egg. So let's put that as a sort of 'stretch goal' for sperm size. The basic goal will be more realistic, at 1mm long, counting the tail. That's small enough to fit in a testicle and through the vas deferens, while also being appreciable to the naked eye. If I can make my sperm measure 1mm, I will be a truly happy man.