I really like this pic :) (tho I think Suika is drawn too big- maybe she has leapt in the air?)
Last I checked, she can change her size fairly freely.
The white 'skirt' on Marisa- I assume that is her petticoat lol? I guess Reimu has raised her skirt- I think that is it kinda bunched up under her left arm? It might just be her apron tho.
It's her apron. You can see her black dress extending past it at the bottom left.