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Non-cartesian charts (where there are no X/Y axes or column/row headers), such as packed bubbles, treemaps, and word clouds, have some interesting behavior.
When size is set using a measure, marks are sized automatically using an algorithm designed to fill the space most effectively. So, there is no control over size in those cases. When size is set using a dimension you will be able to adjust size. But, at that point, the arrangement of marks is no longer packed -- it is stacked (side-by-side).
As Joshua noted, the packed bubbles have a fixed size range to fill up the space. However, in v8.0.0 the algorithm was such that the minimum bubble size was actually rather large. v8.0.1 changed the algoirthm so the minimum bubble size is quite a bit smaller, I set this up in the attached.
Also, Tableau will only generate packed bubbles with a measure (discrete or continuous) on the Size Shelf.The workaround if you want a dimension on the Size Shelf is to add the dimension to the level of detail, then use a measure form of the dimension like MIN(dimension) on the Size Shelf, as in the attached.