On my 11-hour flight from London to San Francisco last week I was seated next to a father and son that were both over six feet tall. At 5'6" my knees were tapping the back of the chair in front of me. They didn't stand a chance.
Aeroplane seats are notoriously small, and unless you're springing for First Class (and sometimes even if you are) you're not exactly going to have a spacious ride no matter where you're sitting.
When it comes to legroom, not all airlines and planes are created equal. Sometimes when you have two similarly-priced flights, one airline is offering you a lot more real-estate than another. The budget airlines down under don't necessarily offer huge differences - though if you're comparing between those and our top carriers, you might find considerable differences.
If legroom is a concern for you (and it probably is for us all), The Legroom for Google Flights Chrome extension can help. Once installed, whenever you do a Google search for flights, the average legroom for seats will show up for all of your search results.
If you're searching for First and Business Class seats the extension will also let you know if you're looking at seats that recline or those that lie flat.
The Extension isn't going to make the seats you find any better, but it will give you a good idea of what you're getting yourself into.