Eleocharis Acicularis supposedly can grow quite tall, but in my experience it doesn't get much over 5" or 6". Eleocharis Vivipara on the other hand can get quite tall, but grows plant nodes towards the ends, so is often trimmed for aesthetics. This might be the one to go for. I believe there was an Eleocharis "japonica" or something like that, but I've never seen it for sale over here, only seen it referenced in 'scapes. Not sure on how tall it grows, but I think it's taller than e. acicularis...