Ace Double D-331 (1958): The Secret of Zi, bound with Beyond the Vanishing Point.

The Secret of Zi, by Kenneth Bulmer. Cover by Ed Emshwiller; 161 pages. Reprint serialization in Britain in New Worlds July to September 1959 as "The Patient Dark." For something like two hundred and fifty years Earth had been dominated by humanoid aliens from the star world of Alishang. But man's spirit refused to be conquered. There was a world-wide underground planning for the day of final liberation. And there were 4 leaders who knew the secret that would guarantee victory - the secret of ZI. Rupert Clinton, intelligence man for this underground, was not one of these 4; yet somewhere deep in the recesses of his subconscious mind, he knew ZI's secret. Clinton, not realizing that he possesses the forbidden knowledge, finds himself caught between his own forces, who fear that he may unwittingly give away Earth's last chance, and the aliens... both sides were anxious to get rid of him, permanently.

(Bound with) Beyond the Vanishing Point by Ray Cummings. Cover by Valigursky; 95 pages. Expanded from magazine version in Astounding March 1931. When George Randolph first caught sight of Orena, he was astounded by its gleaming perfection. Here were hills and valleys, lakes and streams, glowing with the light of the most precious of metals. And, more astonishing than that, it was a world of miniature perfection - an infinitely tiny universe within a golden atom! But for Randolph it was also a world aglow with danger. Somewhere in its tiny vastness were the friends he had to rescue. Captives of a madmen, they had been reduced to native Orena size; to return to Earth they needed the growth capsules Randolph was bringing them. It was up to Randolph to find them - and quickly - for the longer they stayed tiny, the closer they came to passing BEYOND THE VANISHING POINT!

As far as I can tell this is not related to The Girl in the Golden Atom (1923) - though both use a very similar concept: drugs that can make you larger or smaller, and an inhabited world in an atom of gold.