rollup-plugin-bundle-size 
A rollup plugin to show the size of the generated bundle(s).
Installation
npm install --save-dev rollup-plugin-bundle-sizeUsage
JS API
const rollup = require('rollup');
const bundleSize = require('rollup-plugin-bundle-size');
rollup.rollup({
    entry: 'src/index.js',
    plugins: [
        bundleSize()
    ]
}).then((bundle) => {
    ...
});Config File
import bundleSize from 'rollup-plugin-bundle-size';
export default {
    entry: 'src/index.js',
    plugins: [
        bundleSize()
    ]
}