We do lack UI for inspecting what connections a snap would have auto-connected if you installed it and that’s something we want to make easy as it would encourage “exploring” new snaps you aren’t sure about if you can confirm for yourself what interfaces the snap uses without installing it. That in combination with something like my proposal here would also enable trying out snaps which you don’t trust because you could install them inside jailmode with no interfaces connected at all and rely on snapd confinement to trust that the app doesn’t do anything you don’t give it permission to do.
Getting the size of content snaps is probably out of scope for this, but knowing what content snaps would be installed if you installed a given snap a la interfaces that would be auto-connected and have default-providers definitely seem in scope for this.
Also this topic should move to #snapd category and be tagged “backlog”. I seem to have lost my ability to move topics around and tag them unfortunately…