The official Second Life blog has today an entry about so called megaprims (greater than 10m at one side at least). Megaprims where never intended to be, but they happened some while ago and they are in use in different buildings, you can either get them just so or buy them at stores.
They are rumored to have a negative impact on the physics engine and to cause lags and such, but none the less many builders have adopted them very fast and used them in their buildings.
So there's a discussion about it they should stay or not; at the moment they are only tolerated by Lindenlab, but they don't really like them.
So what's the sensible approach? Are they needed or not? I guess they are needed, because they fit into a gap. I mean, why would you use for example 9 prims (10x10m) to make a floor that's 30x30m wide when you just can use one prim? Because they are needed, they've been used and there's a market around it.
But of course too big megaprims are not right, so they should not be bigger than a whole sim, meaning 256x256x256m. In those parameters they could really enrich the building experience.