To be honest I didn't even think about the shipping costs, it will add up but shouldn't be too bad spread out over time. For sizing I would try and stick as close to 10mm as possible to keep a uniform look. The common practice when stringing will be to sort them smallest to largest with the small pearls being in the back and the largest pearls in the front of the necklace, so a little variation is ok. Just keep in mind if you have particular pearl(s) that you want front and center in the necklace they should be the bigger pearls of your lot to look right.
If you plan on purchasing all the pearls online, researching like you are now will save you a lot of frustration down the road. Figure out exactly what you want then just start contacting vendors to see if they have it. There are some good deals to be had on Ebay as well but be careful there are 10x more scams then deals, but they are there if you hunt.
Happy hunting!
