I faced a similar issue when I decided to buy pearls for someone of fairly petite stature. I made an image similar to the one below to compare mm sizes.
The top line is 3 mm circles. The lower line is 6 mm. Although they may not show up at exactly those sizes depending on your computer and monitor settings, the relative size difference will still be apparent. Pardon me while I indulge in a little math
6 mm pearls are not "twice as big" as 3 mm. The cross-section of the spheres would actually be 4 times as large, and total pearl volume would be 8 times larger. And, as you can see, the visual impact is significantly different as well.
Freshadamas do seem to be a good way to go for quality and beauty. Definitely give Pearl Paradise a call and see what they can do for you.