Our nursery is roughly 3m x 4m, pretty well standard bedroom size. We've used it from day one Currently has cot, rocking chair, change table and chest of drawers (has a double built in wardrobe). Would easily fit a single bed instead of cot.

Our other two 'kids' rooms are much bigger, more like 4m x 4.5m - this is really quite huge, they have double built-ins as well. We're planing on 2 kids, so eventually the nursery room will be converted back to a spare/guest bedroom. But we built our house so the reason these rooms are big is that we deleted a small (not big enough to be functional) activity room.