I use Dropbox because it's does leave local files on any PC that you install the client software on. It's synchs files really quickly. The fastest I have seen in storage online services. It's just drag & drop to folder and within a minute it's done.
I don't use the Calibre cloud script. I have mine set up in folders by theme. I do use Calibre to save them to my Dropbox so it uses the plugboard option and the metadata shows correctly on my reader.
I have a Kindle so all I do is log on to Dropbox, type in the little search box on the left a few letters of the book I want and it shows up underneath as I type. No need to browse through all the ebooks I have on it. I love this! The search as you type in one of the reasons I switched to Dropbox.
I also back up my ebooks to DVD and I have a external drive I can grab if I have to. I don't take any chances.
Always have backups of your backups because ebooks are precious!
Dropbox doesn't have a file size limit that I know of. I know if uploaded via their website it's 300MB but through their client it's seems to be only limited to the space you have available.