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Welcome to the Bunnings Workshop community @MarkyMC. It's brilliant to have you join us, and many thanks for your question about constructing a lean-to-shed.
For an exterior structure, it would be best to use H3-treated timber as any water that enters the structure will not cause the treated timber to decay. If the structure is completely sealed from the elements then H2-treated timber could be used.
The size of timber directly corresponds with the weight it's bearing and roof spans. For an average-sized lean-to-shed post, you'd be looking at 88 x 88mm 2.4m Post H3 Treated Pine Laminated LSOP FJ posts. However, there are also 112 and 135mm options if this is a larger shed.
I'm keen to hear more about your project, so you might like to show us a sketch of the plan for your shed. Let me know if you need assistance uploading an image.
Please let me know if you have any questions.
Mitchell
