I have a mature live oak tree in Florida that is much taller than my two story house. I have a small yard and would like to compost around it if possible. I've read here on stackechange that this can be bad for trees, but I'm not sure if that warning is relative to size. For such a large oak tree, would a 5 foot ring of composting around the trunk be harmful?
No, you should not use the area around your tree for composting,and yes, if you do, it will harm the tree. For one thing, a compost pile needs to be minimum 3 x 3 x 3 feet square, preferaby larger, for a more efficient composting process, because this allows some heat to be built up. Clearly, having something 3 feet high all the way round your oak will compromise the health of the tree.
Try to find somewhere else to create a composting area, or get a proprietary compost bin from a retailer - these take up less room in terms of width.