Degi-Key has 25SWG as an obsolete item. While the instructions for a project say to use, "25cm length of 25SWG wire. Wrap the wire around a cylindrical object of 6mm diameter and take it out after eight turns." http://www.electronicshub.org/fm-bugger-circuit/
Additionally, is there a general rule (such as 0.2 mm tolerance) for small inductors (less than 20 turns ~ maybe) for most gauges of wire?