I run a rather large game project on here (NSFW, warning): http://textadventures.co.uk/games/view/xsivcaah2e_pmjkiw1vplq/the-hotel
For some reason, the online save function has stopped working. I've looked at the code and compared it to a game that does save and it's exactly the same, so I dunno why it's not working. I've had problems with the size of the game before re: quest's servers, could that have something to do with it?
Don't know whether or not this is still an issue for you, but I'm also planning a similarly large project (in a similar genre, funnily enough) and was worried about this. A couple months ago I came across a library that saves the game data to a copy/pasteable code that I think looks promising to get around any filesize issues: https://github.com/Leviathon44/SaveLoadCode/wiki
There's a post about it over in the Libraries and Code Samples forum as well (here), but I believe the post is out of date compared to the actual git repo, so I would take the github as truth, rather than the forum post.
It isn't perfect and has some limitations, but it looks like the developer is still updating it as of a month ago, and they've been pretty responsive the handful of times I've messaged them. Was pretty easy to integrate into my existing game too, though I definitely recommend looking over the wiki (it's a bit long, but I found it helpful), maybe you'll find it helpful too?