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I’ve recently hit an invisible limit in data designer: the 16mb payload size limit.
Long story short:
I had a decently sized data design pictured below, 24 tasks and trying to create 4 union tasks to bring my data set together(bringing it to 28 tasks - NOT MY LARGEST DESIGN). The 4th union (circled in red below, and at some point the 3rd one) refused to create itself with an unknown error message.

Turned out to be that the payload size would have been over the limit of 16mb with my 4th union.
YET there is NO indicator that we are approaching the feature limit anywhere in the UI and the error code is cryptic at best and helping no one: not the solution designer and not Gainsight who is getting tickets for things we could self-resolve.
Enhancement request:
We need a visual cue such as a progress bar, in query builder, data designer and rules engines showing us HOW FAR or HOW CLOSE from the product limitations showing both the current size of the payload (a figure, in bites ideally) and the progress towards the limit (the 16mb), ideally from green to red to show good and bad.
Since tasks and field size in the payload varies and can’t be predicted, we need to see that live calculation and progress bar. Besides the need to avoid limitations, I’m certain it would help us optimize our designs, including those that don’t hit the limits but perhaps could be further optimized.
And for those who want to know the end of the story:
The culprit was trying to bring in the time zone of the recipient (required to send emails in the person’s time zone - first task highlighted in yellow) and the sender’s time zone (“users fetch”, highlighted in yellow, because our standard Outlook email signature features that info and we’re trying to mimic the signature for consistency).
Picklists with a lot of values apparently increase the payload drastically, and time zone has 432 system values. After changing the sender time zone to a string (duplicate as string function, I could create the 4 union tasks BUT the design failed to run and error logs indicated this message: Payload document size is larger than maximum of 16793600. At that point, the error message was more helpful. So I needed the two fields but I was NOT ABLE to use both, including with the conversion to string (despite it working up to creating my last union). Using both apparently would require me to nest two designs together, and without proper chaining capabilities… what could possibly go wrong?