FG dress advice

My FG will be 3 on our wedding day. I'm concerned about WHEN to buy her dress. She's tall for her age and is growing so fast. How soon before the wedding should I buy it? Should I buy the next size up just in case? She lives in Florida with my parents so I can't go to fittings with her. I'm also planning on buying her gown from Ebay. There's a company that makes gorgeous FG gowns for so cheap, they look just as good as the ones that cost hundreds. I guess I can buy it, send it to my mother and she can bring it to a tailor for alterations if she needs any???