I'd be surprised, if you can obtain "true" Aponogeton natans, since it is really not very usefull in a tank. It produces allmost no leaves under water, but goes straight to surface.
There are numerous hybrids, from A. natans, though - usually producing leaves under water with more or less curly leaves. Those hybrids could very well be traded as A. natans, I suppose. Size will be unpredictable, depending on which other parent is involved.
- as for A. crispus, the size makes it a possible choise for a 60 cm. tank. It will be a bit "dominant", though. It is possible to continuously trim away the largest of leaves, this way holding the plant back in size a little. A. crispus will flower very easily, once established, usually with white, fragrant flowers.
IF you can find Aponogeton robinsonii (and I think it will be difficult), this is a rather small species, flowering very freely (white, scented flowers). Aponogeton capuroni is also a quite small species, having very beautifull, strongly curled leaves.