I prefer [name_f]Rosie[/name_f] over [name_f]Rosy[/name_f]. I think that [name_f]Rosie[/name_f] can stand as its own name, but I would probably use [name_f]Rosie[/name_f] as a nickname for [name_f]Rosalind[/name_f], [name_f]Rosalyn[/name_f], [name_f]Rosemary[/name_f] / [name_f]Rosemarie[/name_f] , [name_f]Rosalie[/name_f] / [name_f]Rosalia[/name_f], etc.
[name_f]Rosie[/name_f] on its own is adorable (and I definitely prefer that spelling over Rosy), but there are plenty of full names that would give you the nickname if you felt it was too short on its own (Rosalie, [name_f]Rosetta[/name_f], [name_f]Rosalia[/name_f], [name_f]Rosalind[/name_f], Rosalyn/Rosaline, [name_f]Rosaleen[/name_f], etc)