Looking at Crush.pics but wondering how much it will actually improve your site?
The long and short of it is that it varies greatly depending on your website, and there's no way to measure how much it will improve your site.
You can use a tool like Pingdom to get a general idea of a page's loading time, but it will only scan for the specific page you enter.
Your loading time will also vary by location - someone in Australia will have a longer loading time than someone in Canada, but usually just by a few seconds.
Site speed is typically a result of your Internet Service Provider or website server.
To determine an entire website's size, you would need to run Pingdom for each and every page on your site and tally the total of each page size, which would still offer skewed results.
Crush.pics will compress your images but the only way to measure the difference in Site Size (or speed) would be to run complete tests before and after using the app.
Google PageSpeed Tool
Google's PageSpeed Tool is a great resource for website developers but using it with Shopify is a bit more complex. Even after compressing your images, it's possible that Google will say that images need to be compressed.
This is easily fixed in Shopify - See this article