[lauralynne]

ask her to picture her shiney new bike on the ground because it's likely to end up there (a few times). With a used bike, that's not so heart wrenching. She'll need to be able to pick it up herself too.
She won't outgrow her bike within a year unless she's an anomoly - even a 250 or a 500 will take a good year and a lot of miles before you're using it to 100% capacity.

Meanwhile, have her go sit on bikes - any of them, all of them.