[timarie]

I know that a B cup already fits, I have many of them, I could wear them even as I was much smaller, I was thinking more about how they look like naked, they always look bigger in a bra, especially if padded or lined.

So, based on this, you DO know what a B cup looks like on you naked if you currently fit a B cup. Just take your bra off and wa-la.

And for the purpose of imagining cup sizes on you, don't buy padded or lined bras, obviously, unless they are only lightly lined (lightly lined bras-- although they altar the shape of the breast-- don't add to the size of the breast; they only provide enough fabric for modesty purposes). That is a pretty obvious one!
The bra method is the closest image that I am able to get to imagining a size on me. No matter what you do, you will have to imagine. So if imagining with a tape measure is more your thing, more power to you! I just personally find it easier to imagine a bust size on my chest from a bra. I know how my breasts look naked, and I know how they look with 32C bras on me. Therefore, I can imagine fairly easily how my breasts would look naked if they fit 32D's, as I can see the difference between the 32C and the 32D on my chest, and I project that difference onto my naked breast for the purpose of imagining the 32D naked breast. In the end, it is all an imagining game, but making it to fit YOUR body is always the tricky part! I hope you find something that works for you