About 7.8 inches inside, 5.5 inches outside. Closed like a passport, open like a trackpad
After cross-checks, size is the stablest block. Apple made it short and wide on purpose, not another tall Android foldable.
The cover is about 5.5 inches. The inner display is about 7.8 inches. Closed proportions were compared internally to a passport. Open proportions were compared to a Magic Trackpad. That is a design choice: Apple wants a first-glance difference from Galaxy Z Fold, using short-and-wide as the identifier.
7.8 inches is a little under the 8.3-inch iPad mini, but large enough to move split view, drag and drop, and near-tablet reading onto a phone. Gurman thinks it will be strong for games, and fine as a small tablet that still fits a pocket.
The cover is not decoration. Closed, it is the main surface, close to a regular iPhone. Apps have to serve passport mode and trackpad mode, not stretch a slab layout once and ship it.