Wearing implants may not be a good indicator of what size you should/shouldn't be. For one thing, they won't fit the same in your bra (regardless of bra size/fit) as your breast will fit inside your bra post-op.
Perhaps one alternative would be to get a pair of breast forms, such as can be found here.
You could probably find other sites online which provide similar solutions. In order to accurately determine their exact volume, you would need to plunk one into a full container of water, and measure the amount of water which spills over the edge. A bit tricky, but quite accurate. Be sure to submerge the form completely.
The advantage of wearing a breast form is that it should have a more natural curve which better fits against your existing breast. The material will also more likely 'stick' to your skin better than the silicone shell of any breast implant. What you want is for the overall shape (ie, your breast plus whatever else you stuff into the bra) to be a more or less coherent/unified shape, rather than a bulge sitting on top of your breast.
Now you've got me thinking. I know a woman who has a gf with some skills who (for reasons I no longer remember) did a mould of her chest, then ended up sculpting something else. The result: a pair of breasts which would make her look like she was sporting 1200cc implants. I should ask her to see if she can check with her friend to see how a complete newbie would go about getting this done. Even if it means hiring someone with the appropriate skills. I'll try to remember to ask her about this.
Meanwhile, also consider what bra you'll need to wear to pull this off. With the wrong bra, you might find the straps digging in uncomfortably, or worse, a clip on the strap positioned right on top of your shoulder, sharing all manner of discomfort.
Hope this helps.
Master "might as well just round up to 1000cc" Dragonfly