LibSize tells you (approximately) how much a package will add to your bundle size.
libsize.com also exposes an API:
https://libsize.com/json?url=https://unpkg.com/react@15.5.4/dist/react.min.js
Give the URL to weigh as the query parameter url. This is what the UI uses.
This is the very first version of Libsize that I threw together in an evening. In the future, libsize.com will accept Github repo paths and npm package names.