[Speedslug]

Remember that this is a gas pressure pushing against the runniness of the molten glass that determines the size of the bubbles.

Hotter, runnier molten glass will allow the gas to push the bubbles out more while cooler, thicker molten glass will resist that push and limit the growth of the bubbles.


An other thought on figuring out how much baking soda to use;
I would take that tiny amount ESC mentions and scrape it into a very fine line to get it as thin as possible on a marver and then roll the molten bead over just the smallest portion of it and encase that.
Start with as small an amount as you can and work up from there.