I've had breast relief hundereds of times. Here are a couple of options.
1. She massages your cock between her breasts until you come. For this she'd need decent-sized jugs.
2. You masturbate onto her breast (my much preferred option). My favourite breast is firm and tipped by a nipple which I'd sucked hard. I don't want the breast to be too big as it's just cumbersome and heavy.
What are you thinking of?