The Crucible

From the introduction:

When the Adventurers decided to assist the sage Baerhoff with his “little alchemical difficulty,” they didn’t realize what they were agreeing to. With a spray of chemical fluorescence, the party finds they have been shrunk to nearly microscopic size, and inserted into the good Doctor’s device in order to “work out any problems!” To regain their size, Baerhoff, now a looming giant, pronounces they’ll have to figure out what’s gone wrong with his experiments...from the inside!