Looks like 3/8, possibly 1/4, mip actually. Thought I had some 3/8 fittings on hand but if I do....they've grown legs. But, they simply unscrew from the body like in the pic. If you're not in a rush I can take mine to Lowe's tomorrow or Sun (more then likely Sun though) to verify. Or, take yours seperated like this to the plumbing area of Lowe's. They have both sizes in the push to fit slots there.
View attachment 250062