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Welcome to the Bunnings Workshop community @homebuilders. It's terrific to have you join us and many thanks for your question on building a picnic table.
To give you accurate advice on screw and bolt sizing, we’d need a bit more detail about the design you have in mind and the calibre of timber you’ll be working with. Picnic tables can vary quite a bit in construction, and the fixings need to be matched to the timber dimensions to ensure everything is strong and safe. If you have an inspiration image of the style of table you’d like to build, that would be really helpful. I can assist you with uploading a photo if needed.
The size of the timber is important because it determines how much bite you need from screws, and how long your bolts need to be to pass through and secure pieces together. For example, with thicker stock you might use M10 or M12 cuphead bolts for the main structural joins (such as attaching the legs and braces to the seat and tabletop frames). These bolts are commonly used for picnic tables because they give a strong, through-bolt connection and the domed head looks neat and prevents snagging. For other areas, 75–100mm galvanised screws are often used, depending on the timber thickness.
Here's a helpful guide: How to build a picnic table. In this guide, they've used 75mm Bugle screws and 100mm Cuphead bolts.
Please let me know if you have any questions.
Mitchell
