Learn French: Il a creusé un fossé de deux mètres de large.

This is the standard French structure for describing physical dimensions directly after a noun. The formula is: [Noun] + de + [Measurement] + de + [Dimension]. Here, it connects the noun un fossé to its measurement (deux mètres) and then specifies which dimension is being measured (large).

Could I use the noun largeur (width) instead of the adjective large (wide) at the end of the sentence?

Yes, both are perfectly correct! When using this specific structure with the second de, you can use either the adjective (de large) or the noun (de largeur). The same applies to length (de long or de longueur) and height (de haut or de hauteur), but for depth you usually use the noun (de profondeur).

If I wanted to say 'The ditch is two meters wide' as an independent sentence instead of a noun phrase, does the phrasing change?

Yes. When using a verb to describe the dimension, French typically uses faire rather than être. You would say Le fossé fait deux mètres de large. If you do use être, the structure flips slightly to: [Subject] + est + [Adjective] + de + [Measurement], making it Le fossé est large de deux mètres.