Its the known feature as of now there is no workaround for this Tableau team is still working on it for independent resolutions for each dashbaord
Hi Tyson, just to add on to what Naveen has said.... I understand what you're looking to do so I jumped in to the Ideas space to see if I could find this listed. I didn't have much luck, but if you have time I would definitely recommend going in to see if someone has already compiled this Idea for consideration (if so, it's worth up-voting it so that it gets more visibility)....
[Program Manager - Tableau Community]
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If you publish your workbook without "Show Sheets as Tabs" checked, the dashboards will retain their individual sizes.
But, you'll need to update your Actions to use the url for each dashboard, since the dashboard will no longer be in the same frame without "Show Sheets as Tabs". You can use relative or full urls in the Actions.
Thanks Carisa Chang for the suggestion. I can use this idea until Tableau gets this ability in future releases.
I might be missing something but can you use Fixed size of 1300 X 1080 for your summary dashboard and min 3200*1080 -Auto for the detail one? With this, even though the I-frame retains the largest dimensions, the visualizations in the summary dashboard won't get stretched out or look out of place as the dashboard has fixed dimensions. Again, I'm playing dumb here so ignore this if you have already tried it out.