The breadth of the entry (literally, opening) of the gate, ten cubits. Obviously this measurement was taken from north to south of the gate-entrance (Ezekiel 40:6), and represented the whole breadth of the doorway and the threshold, or one-fifth of the entire length of the gate-building.
The second portion of the verse, the length of the gate thirteen cubits, is explained by Bottcher, Hitzig, Havernick, Keil (with whom Plumptre agrees), as signifying the length of the covered way from the east entrance, since it is supposed the whole length of forty cubits (the length of the gate without the porch) would hardly be roofed in; so that assuming a similar covered way of thirteen cubits at the other end of the gate-building, as one came "from the house," there would be an open space, well, or uncovered courtyard, of fourteen cubits in length and six broad, enclosed on all sides by gate-buildings.
The roofs extending from the east and west would be supported on the "posts" of the chambers mentioned in Ezekiel 40:10. Smend, however, infers, from the windows in the posts within the gate (Ezekiel 40:16), that the whole extent was roofed in, and accordingly can offer no explanation of the clause; Kliefoth and Schroder prefer to regard the thirteen cubits as the height of the gate, although the word translated "length" never elsewhere has this meaning.