I think it depends on a few things... for me it'd be how long it's going to end up being if you do it in one piece, and whether you could keep your own momentum going with multiple parts.
Like you said, if it's TOO long, then doing it in one piece is hard on the reader. However, personally, I have a hard time doing part of something to the point where I can release it, then picking up again where I left off (see my morbidly neglected storyline).
Perhaps something in the middle? If you split it into just two pieces, maybe it'd be less like episodes and more like, say, an intermission.