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Footer sizing is not a problem if you investigate and plan carefully.
What does the local code call for ?
What is the soil type ?
How far down to bedrock ?
You can buy treated cardboard footer forms quite cheaply in various sizes. They come in both conical and pyramidal shapes. The larger the footprint of the footer makes it less susceptible to settling.
There are multiple ways to fasten your posts to the foundation discussed at length here on the website. Use the search engine.
Use some crushed rock under the footer for drainage.
Deralte (aka Emmett)