Largest non-hot gas giants
According to the models of Mordasini et al. 2012 it is about 13 Earth radii (about 1.2 Jupiter radii) for gas giants older than a billion years.

Younger gas giants that are still contracting significantly can be bigger. How much bigger is wrapped up in uncertainties in the formation mechanism and the core mass and composition.

Gas giant mass-radius relationships from Mordasini et al. 2012. The colour coding corresponds to the fraction of heavy elements in the planet. The larger planets have smaller fractions of heavy elements.