See T247206 and T247573. It seems one can theoretically confirm an instance resizing in Nova, yet Horizon has a different policy that disallows it:
modules/openstack/files/queens/horizon/nova_policy.json: "compute:confirm_resize": "!", modules/openstack/files/queens/nova/common/policy.json: "compute:confirm_resize": "rule:admin_or_projectadmin",
Any such distinction would presumably be a problem for opening up the API directly for use.