Ray Cummings 
Beyond the Vanishing Point 

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 madman, 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!