a-usage.js

<ElementQuery sizes={[{ width: 350, size: 'small' }, { size: 'big' }]}>
  {(matched, ref) => (
    <div ref={ref}>
      {/* `matched` is the object that got matched, it's the literal
       object from your array above so you can put anything into it.
       The last item will be matched by default if nothing else does.
       Also, `matched` will be null on initial render since it doesn't
       have the size and it's up to you to render what you want in that
       case.*/}
      {matched
        ? matched.size === 'big' ? 'This is big' : 'This is small'
        : "I don't know yet"}
    </div>
  )}
</ElementQuery>;

ElementQuery.js

import React from 'react';
import lively from 'lively';

// Global registry and resize handler

let _registeredInstances = [];
function register(inst) {
  if (_registeredInstances.length === 0) {
    window.addEventListener('resize', onResize);
  }

  _registeredInstances.push(inst);
}

function unregister(inst) {
  _registeredInstances = _registeredInstances.filter(i => i !== inst);

  if (_registeredInstances.length === 0) {
    window.removeEventListener('resize', onResize);
  }
}

function onResize() {
  _registeredInstances.forEach(inst => {
    inst.setState({
      matched: findMatch(inst.element, inst.props.sizes)
    });
  });
}

function findMatch(element, sizes) {
  const rect = element.getBoundingClientRect();
  const matched = sizes.find(size => {
    return (
      (size.width != undefined && rect.width < size.width) ||
      (size.height != undefined && rect.height < size.height)
    );
  });
  return matched || sizes[sizes.length - 1];
}

// Component

function ElementQuery({ props: { children }, state: { matched }, inst }) {
  return children(matched, el => (inst.element = el));
}

export default lively(ElementQuery, {
  getInitialState() {
    return { matched: null };
  },
  componentDidMount({ inst }) {
    register(inst);
    return { matched: findMatch(inst.element, inst.props.sizes) };
  },
  componentWillUnmount({ inst }) {
    unregister(inst);
  }
});

Thank you very much for this; I also just want to note that https://github.com/d6u/react-container-query has a very similar API