Recommend that you either spring for a couple of prosthetic breasts which match your goal size, or make yourself a couple of fakes, using the rice technique, or Play-Doh (wrapped in cling film to keep clothing from getting stained), or similar.
Buy one or two bras in your guesstimated size (maybe one cup larger vs one cup smaller, for variance), then put everything together / put it on. You'll probably want to find a top (sweater? hoodie? as a stopgap) that you can easily put on.
Then, kick back at home like that for a few hours. Watch TV, do kitchen stuff or light chores, that sort of thing. If you feel daring, go out for a drive somewhere.
In a nutshell: get a feel for what it'll be like, in advance. At the very least, it'll help you to better select a final bra for immediately post-op, as well as having a top you can wear immediately after surgery.
Source: wife and I took this approach; it worked out quite well.