Another way to do this is to create a camera, render to texture (make sure the camera has aspect of 1 and is a "nice" resolution like 512x512) and then move the camera to cover the playable area in increments of the camera size. While you're doing this save a screenshot using the same code in that script MetalGreg posted. You'd have to stitch it back together in the end though, but you can do this in your favorite image editor.
Another way you could do this is to create a downsampled tilemap spritecollection, swap the spritecollection and that'll resize the tilemap to be tiny (eg. replace each 32x32 tile with a 4x4 simplified one). This will give you full control over each tile which could be a lot better than the previous suggestion. You can simply drag the original sprite collection back in after youre done with this.