This is a temporary issue due to Ancestry maintenance. It's nothing to do with a deficiency in FTM, which is perfectly capable of handling a tree of your size. My tree is only about 3,300 people, though with 4-5GB of media, but other people have huge trees with tens of thousands of individuals - some over 100 thousand I believe - and they have been successfully syncing until this current problem that started a few weeks ago.
I know it must be very frustrating if you've just started to use FTM at this time, especially if all your research is already on Ancestry and you can't yet download it, but once syncing is working again I hope you'll find that it's a powerful and flexible tool for genealogy. Inevitably, it also has quite a few quirks, bugs and deficiencies, but that's the case with virtually all software of this level of complexity.
When reading this forum, bear in mind that most people only post here because they have a problem or a question, and not when everything's working fine as it does most of the time!
I know it must be very frustrating if you've just started to use FTM at this time, especially if all your research is already on Ancestry and you can't yet download it, but once syncing is working again I hope you'll find that it's a powerful and flexible tool for genealogy. Inevitably, it also has quite a few quirks, bugs and deficiencies, but that's the case with virtually all software of this level of complexity.
When reading this forum, bear in mind that most people only post here because they have a problem or a question, and not when everything's working fine as it does most of the time!