Wikipedia's page about gas giants says so. The paper where the computations are done is arXiv:0707.2895. I haven't read the paper in detail, but the figure you are interested in is figure 4 (p21), where you can see that the maximal size for a planet is roughly 1.5 Jupiter radius, for a planet made of 2000 $M_\oplus$ if hydrogen.
Beyond this mass, the pressure augmentation at the centre is enough to shrink the metallic hydrogen inside the star and increases the density enough in order to reduce the global size.
Edited after reading the paper: If you want to read only a part of the paper, read §4.1.1. which details a lot of things without being too technical. The computations are made at 0 temperature, and they postulate that the extra-solar planets (slightly) above the H-curve in fig. 4 are due to thermal effect. Also of interest is the fact that they stopped their calculations at 4000 $M_\oplus$ (13 M_J), because deuterium fusion is ignited above this mass, transforming the planet into a brown dwarf. However, Wikipedia says that brown dwarfs radii is approximately constant, so I guess that the thermal affect are still small for them.