I will determine the tube size based on the availability of compression fittings... I would like at least 1/4" ID, larger if available. I don't want to wait all day for the beer to flow but I also don't want to disturb the bottom to much when draining into the keg. Any reason why copper wouldn't work well for a racking tube or are there any oxidation issues with it sitting in the fermenter for 4+ weeks? I don't think it's an issue.
You will likely be fine with copper, but there are some concerns about galvanic corrosion due to the different metals. I would go with a 1/2 dip tube and order a short stick of stainless from McMaster #8989K872.