The tray dimensions are unrelated to the tub dimensions, other than as Old.Tony pointed out regarding the mounting points. The very vast majority of trays are longer than the tubs. When you see photos of them just have a look at how far they extend over the towbar. Because I wanted to keep the overall length of my Navara a bit shorter I made my tray 1650mm long but I've never seen a commercially made tray that short (for a dual cab). They generally start at 1800mm and go up from there. The width is typically around 1800mm, as is mine.
It's generally it's not too much trouble to adapt a tray to a chassis. Most commercial trays are not actually made for a specific vehicle but are instead made in such a way that they supply a mounting kit that lets you fit the generic tray to your make and model. So, if you know what the make of the tray you're interested in is, I'd suggest contacting the manufacturer and asking if they have a mounting kit for your Navara. Don't forget to mention what cab you have, which you neglected in your question here.