picking sizes for post-op clothes

so I'm thinking about getting myself some button-down pyjama tops for during recovery but it's just occurred to me I have no idea what size I'll be afterwards

my chest is a US I-cup at the moment so it's gonna be a huge change getting rid of it all and clothes are gonna fit wildly differently so ... what do I do? do I measure myself just above my chest where it's flat and use that to guide me? or do I just go a size or two down from what I normally wear (since I currently have to buy things oversized so they close over my chest)

thank you in advance <3