The way to think about this is whether wearing a 36 or 38 bra or a shirt size that's too big for your upper body will "balance out" or "overwhelm" your upper body. Currently you look like a full A cup size (in a 32 inch chest). If you get an implant whose width matches the width of your breast and it's a high profile gel in Mentor or Allergan (or a high profile saline-filled in Allergan), then you will look like a 32 full C and still look natural in the sense that the implant will fit and look like a breast rather than a breast and an implant. If this size seems to fit your conception of proportion and desirable size then it should work for you. In general, as long as you don't cross into the D cup range (which you can't reasonably do in one operation) then you can still look proportionate. If you are taller, you can handle a larger breast size and look proportionate, but ultimately you should go with what makes sense to you as long as it's feasible.
I'm guessing at the width of your breast based on experience and your 32 inch chest around, but I'd guess it's about 12 cm or a bit less. This would be about a 280 or 270 cc implant in a medium profile and maybe around 380 in a high profile which, if saline, could be filled to 400, but in general a 400 cc implant in less than a high profile will not predictably fit and will not look natural. You wouldn't just pick a shoe size or dress size that you like and hope it fits and looks natural. You'd be measured to see what would fit you. And since you can't "try on" a breast implant realistically, you should go with what is known to fit you and what effect that would have on your apparent breast size rather than choosing an implant volume.