Due to request from support, after checking with our KB, we didn't have an explicit support for this on either documentation or codebase:

From earlier Magnolia versions I know a way via model class...

PropertyUtil.getLong(AssetNodeTypes.AssetResource.getResourceNodeFromAsset(node), AssetNodeTypes.AssetResource.WIDTH, 0l);

But this is slightly complicated. You need your own model class and so on... Is there simple way to retrieve the image width/height directly inside FTL template?

Workaround
Using damfn you can get the Asset object which has a getMetadata() method. The MagnoliaAssetMetadata has both getWidth() and getHeight() methods.