Best bottle nipple size for breastfed baby?

My 1 month old currently takes a bottle once a day with a Newborn nipple. I'm looking to buy more nipples and am confused--- slow flow nipples are for breastfed babies but the flow is faster than the newborn nipple which seems fine to me... But I'm a FTM so I really have no idea. Which size is more realistic for a breastfed baby?