Sure enough, you can open the Export PNG Image dialog, select custom and type the coordinates manually. But that's crazy.
You can also use the Page export, but that means you'll need to re-adjust the page if you're exporting different images from the same .svg file. Also, again, adjusting page to include proper margin and alignment is extra work.
What I usually do is that I create transparent rectangle and I include it in the selection when exporting, then I select Export->Selection.
Those invisible rectangles are easy to lose. Is there another way of exporting set rectangular area (eg. 64x64) without using any helper rectangles or the page size?