Ah I understand now, thanks for the explanation. I would suggest doing this by downscaling some of the images that you use in Spine. That way the images you pack are already the size you want.
If you are using the latest PhotoshopToSpine script you can do this by using the tag [scale:number]
in your layer or group names. For example, using [scale:0.5]
will create a PNG from your Photoshop layer that is half the size as it was in Photoshop. It will also scale the attachment to twice the size, so that it still appears with the right size and position in Spine, just with lower resolution.