The biggest known planet in the Milky Way may be HD 100546 b, which is a very large gas giant in the process of forming with a diameter roughly 6.9 times that of Jupiter, or 77 times that of Earth.
The radius measurement is highly uncertain, as some of the material surrounding the planet may be masquerading as being part of the planet itself. The largest planets whose sizes are known for certain are HAT-P-67 b and XO-6b, both with diameters around 2.1 times that of Jupiter. Both of these planets have had their diameters measured directly as they transit their parent star.