"too large" only shown after uploading

Trying to (force-)sync an oversized collection returns an error only after upload has already completed, when that could easily be checked (on both sides) before starting.

[Dunno how it’s handled internally, but I could picture that the server might use separate repeated checks anyway in case somebody malicious tries sending multiple GB.]

It’s something most users will never hit, so not currently at the top of the priority list.