As far as I know it is relay dependent. Some relays may allow larger messages than others. Some months back I created a message that was many K long. I don't remember the actual size but it made it onto the relays we were using at the time.

>From: bigmarh<-DerekRoss at 04/25/23 12:40:11 on wss://relay.damus.io

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>Is there a spec on the Max Size of event content that relays will take? I can't seem to find it spelled out anywhere.