I don't have any experience with really long notes, however what sprang to mind was Imapnotes2 for the reason that it syncs notes to an email accout, and seems to treat notes as emails as I can look at them in my mail client. This may not work however, it would need to be tested for your needs. I use that as my notes app and wasn't able to test myself before posting because searching "a crapload of plain text" and the likes, to copy and paste, wasn't as yeilding as I thought it would be. It can't hurt to try it out though. Prior to using Imapnotes2, I was using No nonesense notes, which can sync either to a local file or to a google account. The sync to Google is much slower, and battery hungry as a result. I always used a local file for that reason, and I don't remember where the data went or how it was handled. I don't know if there is a limit on size with that app either. I think, though, that between these two, its worth giving them a shot and pasting in something you already have to test them out with. They're both available on the Play Store and on Fdroid and (I think) both treat notes similarly.
A workaround would be using a folder in your email account. Its quite possible that Imapnotes2 can be used in conjunction with this method, and if all else fails you might try k9mail and set it up to sync only your notes folder and turn off polling (the push mechanism it uses). Using k9mail might be a pain to set up properly.