The light usage scenario would add up to around 150 MB per month, so you want to choose a plan of about 200-250MB if you only plan to view a few news sites and send a few emails each day.
The medium usage scenario assumes viewing the same regular news sites and eBay browsing as the light scenario, but doubles the number of emails sent and received and adds watching a YouTube video and doing one search with Google Maps. This comes out at about 11MB per day, so you’d be looking at a plan with a cap around 400-500MB per month to be sure.
A heavy user in our scenario could double all of the above, but view three YouTube videos and do four Google Map searches to peak at around 28MB per day. With this usage, you’d probably be safest going for a 1GB data plan to avoid excess charges.
To help save on data, you can sometimes browse a web-optimised version of a site. It usually has a different URL (such as mobile.smh.com.au, the Sydney Morning Herald’s mobile site) and the differences in data usage can be significant. Note that some websites can detect if you are using a smartphone and will load the web-optimised site automatically, but otherwise look for a link to the “mobile” version on the site.
The bottom line is, you can see from these figures that plans with a tiny quota are probably best avoided if you use the internet on your phone even lightly, as you will likely end up paying more in excess usage charges than upgrading to a higher plan.