If you are using the composite beams then it is best to stick to the footprint size, this will then give you a nice lip over the base and this then acts as a drip, there will be zero chance of water getting under your base.
If you were using the timber profiled beams then a slightly larger base would be necessary. With the composite foundation beams this is not required.
Please though make sure your base is 100% accurate when making to the footprint.
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