What would be considered the most useful sources for market data particularly when researching industry size, value etc?
In Australia, the ABS has some great stats. Some you have to pay for others not. Their help desk staff are awesome at helping you mine the information you are searching for too.
Industry organizations will generally have good stats around their members too - even if they are not 'formalised' . Sometimes making the call and asking is a great place to start.
Were you looking for something in particular?
Jef Lippiatt , Owner at Startup Chucktown
Well it depends if you are talking directly about the information or about tools to evaluate that information. Here in the United States there are lots of public documentation available if you are willing to track it down. Much of the information you mentioned would be located in Census data. Now, it must be taken with a grain of salt because typically Census data is only collected every 10 years.
Also another great place to look is just some searching on your industry for whitepapers or market reports.