Welcome to the Bunnings Workshop community @LanhNgo. It's wonderful to have you join us, and many thanks for your question about building an extension to your house.
You might need to consult with an engineer to draft up some plans for you. There are many variables at play when deciding on what timber you need to use. Is this going to be an enclosed sunroom or more like a pergola? Will there be a roof on it? Here are some pergola span tables from Hyne timber for you to reference. If it is a pergola, then you'll find this step-by-step guide useful: How to build a pergola frame.
As you can see from the span table below, with no ceiling and 900mm centres, a 140 x 45mm timber will span 3800mm.
Keen to hear more about your project and assist in your timber selection. Please let me know if you have any questions.