Flat size / finished size
Flat size is the trimmed piece opened out; finished size is what remains after folding and final trimming. A4 folded once finishes at A5.
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What it is
Flat size is the width and height of the piece laid completely open after trimming but before folding. Finished size is what the customer holds after all trimming and folding. An A4 sheet folded once in half has an A4 flat size and an A5 finished size, and the standard DL leaflet is that same A4 flat size finishing at 99 x 210mm.
Put both dimensions on every folded-piece brief because each controls a different decision. Flat size determines sheet fit, artwork extent, and cost. Finished size determines envelopes, racks, mailing limits, and how the object feels in the hand.
Gotcha: flat size normally excludes bleed, and finished size is not the same as the width of every panel. A roll fold needs narrower nesting panels, so three equal divisions can buckle even when the outside dimensions add up. State width before height, label the units, and add a panel diagram instead of leaving either size to guesswork.
Ask AI for it
Set up this folded piece in Adobe InDesign with separate labeled dimensions for flat size and finished size, written width by height in millimeters. Make the document page equal the trimmed flat size, add 3mm bleed on all four outer edges, draw fold lines and panel widths on a nonprinting technical layer, and export a PDF/X-4 proof that shows an outside view, an inside view, the folding sequence, and the final closed orientation.