Photos: Left:Dhiraj Mallick, Vice President, Engineering and Business Development, Cerebras Systems, hold the world's largest  computer chip, the WSE-2. Right (top):  Currently the tiniest microchip in production, the IBM Power 10  made by Samsung has18 billion transistors on board. Right (bottom): The implantable chip developed by Columbia University is 0.1 mm cubed  in volume and is seen  in a magnified view, on the tip of a hypodermic needle
September 13 2021: The first half of 2021, has seen announcements at two extremes of the microchip manufacturing spectrum. 
A, US Silicon Valley-based Artificial intelligence company, Cerebras Systems recently unveiled the second edition of its Wafer Scale Engine ( WSE-2), which is the size of a dinner plate -- and the world's largest computer chip.  It packs in 2.6 trillion transistors each about 7 nanometers wide ( one nanometer is one billionth, that is 1/1,000,000,000 th of a meter.)  configures them into 850,000  computing units or cores that woul typically work together on AI applications. The logic for packing so many cores into a single slab of silicon is that otherwise, researchers would have to combine that many separate processor chips adding to interconnects and delays. Recent New Yorker article on WSE-2 here
The smallest microchip  commercially available today,  is IBM's Power 10 processor, released in August last year and made  for IBM by Samsung. It also uses 7 nanometer technology and packs 18 billion transistors into a chip  is about  2.5 mm across and  fits on a finger nail.
But smaller chips are  on the way. A few weeks ago, scientists at Columbia University, New York, announced that they had developed a microchip that is 0.1mm cubed  in volume and is small enough to be implanted into a human   using a hypodermic needle and can measure  body parameters.  For example a surgeon  performing a critical operation might inject a few such microchips into the patient and then use an ultrasound scanner to  communicate with the chips to check on the status of vital parameters. The researchers describe their work in  Science Advances.