You can use your small images in Spine and larger ones at runtime so you don't really need to do anything.
Something like this should do it:
scale = runtimeSize / spineSize
json.scale = scale
And then load the correct atlas.
You can also take a look at rstarks rstarks Spine scale utility
Nate just added scaling to import:
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This is what happens when you use it and don't have larger images:
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This feature will be in the next update.