Aside from the nice link about Ancient Volcanos the cogitator posted, Crater Lake has a six mile wide caldera, and Pinitubo has a 3 mile wide caldera. Do we know how wide the Thera crater is?
Other major eruptions not listed in the Ancient Volcanos link include: Sakara-Jima in Japan which left a 15 mile diameter caldera about 22,000 years ago. The current volcano of that name sits on one edge of the caldera bay. Two more major eruptions of Yellowstone, the most recent about 640,000 years ago, and perhaps coming due soon. There were also major eruptions of Vesuvius in prehistoric times leaving a caldera that the current mountain sits within.
Quick searching without in-depth reading found a value of about 4 miles. I think the volume of the caldera might be more indicative than the width related to the size of the eruption that created it; both are dependent on the size of the actual mountain that collapsed.
Other big calderas: Yellowstone (obviously); Valles Caldera in New Mexico; Katmai Lake in Alaska; Lake Toba on Sumatra.
Nice post. The caldera of Santorini is prehistoric (10s of 1000s of years old, at least) and obviously other ancient craters represent much larger eruptions. :’)