I guess it would depend on what you would like it to look like. I used an 8mm pearl on the wedding cake in my pics w/the black fondant band. It looked real and but did not take over the cake. Actually I have had so many people think they were real. These were actually Jennifer Dontz's chocolate pearls from her DVD. They were easy to make once we got the hang of the mold but had a huge impact! Oh, and the sizes were 12,10,8 & 6 w/6-6" on the bottom.
I did use 8mm on all of the tiers. I used super pearl to dust the mold and also to "buff" them when they were out of the mold. I used a small soft brush to apply the super pearl to the mold and a slightly bigger soft brush to "buff". Once we got the hand of using the mold, we could rock them out pretty fast!
I only use the full bead mold. They are really easy once you get the hang of it. What type of problems are you having? Maybe I can help.
What I did was find a pearl necklace that I had to see if it was the right size I needed. My father had given me a lot of costume jewelry when my mother died so I had a few different sizes to pick from. Then I bought at Michael's the Amazing Mold Putty (use the coupon to get half off) and I made my own pearl mold. It was so easy and fairly inexpensive and I was able to make a few different sizes then paying $25 or more for one size mold.
I don't know anything about Amazing Mold Putty. Is it food safe? I looked into another brand at HL and it wasn't. That would be my only concern. There have been many things that I would like to try to mold but have not because of this. I know that there is a food safe one on the market that advertised on here. It is still on my wish list for when I get some extra money together to play with something new! 
Found this on the Amazing mold putty website.
"Can I bake my cured Amazing Mold Putty mold?
Yes. Amazing Mold Putty is food grade compliant up to 395 degrees F. This enables you to either bake food, polymer clay, or any other type of material in the mold without harming it.
If you plan to use the mold for food, do not use it with any other material before putting your food product into the mold to eliminate any possibility of contamination.
The Amazing Mold Putty is a very good insulator of heat, so it is highly recommended to preheat your mold as you preheat your oven to provide a more even bake process when baking food. Otherwise, your food product may cook on top but because of the thickness of your mold, the heat will take much longer to reach the food in the mold cavity and make for an unevenly cooked product."
Sounds food safe to me! I need to get some of this stuff for a sugar bottle mold 
Awesome! Thanks for sharing! Something to add to my Christmas list.....yeah! That could open a whole new world......
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