The Moon is 14% wider at its nearest than at its farthest, but a supermoon is only up to 8% wider than an average full moon, which you cannot see without the two side by side.
Apparent width goes as one over the distance. The Moon’s distance runs from 356,400 to 406,700 km, so its width runs from 33.5′ to 29.4′, a ratio of 1.14. The average distance is 384,400 km, so the nearest full moon is 384,400 ÷ 356,400 = 1.08 times wider than average, and a typical supermoon, at about 360,000 km, is 7% wider. It is also up to 30% brighter than a micromoon, from the inverse square of the distance.
“Supermoon” is a convention, not an astronomical term: the line is drawn at the average closest approach here (363,300 km), other lists draw it a little differently, and the count in a year changes with the line. A full moon looks bigger low on the horizon only because of how the brain judges size against the landscape: measured, it is the same width high or low.
Estimate with the rule, then check it against the calculator that models it properly.
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