Welcome to the Bunnings Workshop community. It's a pleasure to have you join us, and thank you for sharing your question about building a deck.
I recommend having a look at this guide - How to build a low-level deck by @Adam_W. This guide will show you what you need to take into consideration when planning your deck. Many factors affect the cost of your deck and it's important to be familiar with them so that you can price your decking effectively.
Your framing timber size is dependent on several factors. How much space do you have below your deck? What kind of footing will you be using? What pattern will you be putting the decking panels in? I suggest getting pen and paper and making a basic deck frame plan. From here you'll be able to decide in which direction the decking panels will go in.
It's always a good idea to visit your local council and find out what the rules and regulations are when building a deck in your area. There are rules on how far a deck should be from a fence, Rules on how high your deck should be when close to a swimming pool. You'll also be able to find out if your deck size will need a permit.