HD 100546 b is the largest known exoplanet, at 6.9 Jupiter radii.
The NASA Exoplanet Archive does list a radius of 6.9 Jupiter radii for this planet or brown dwarf, but if you look at the original discovery paper, you'll see that this is the radius of the directly imaged emission area around the planet, not necessarily of the planet itself. Applying Occam's Razor, the size of the emission area is far more likely to be caused by a circumplanetary disk, rather than a planet or brown dwarf almost as big as the Sun (which as far as we know is impossible); this is also stated in the paper.
So what's the real largest exoplanet? It could be HAT-P-67b. This planet has a reliably measured radius of 2.085 times that of Jupiter from the transit method; it's a hot Jupiter orbiting very close to its star, causing its radius to be inflated from the heat.
Tau Ceti has 8, 10, or even more planets, several of which may be habitable.
Tau Ceti has four planets that are generally considered "confirmed" - Tau Ceti e, f, g & h - although their existence has been disputed. There have been several other candidate planets proposed in the system, as well as planets predicted based on an equivalent of the Titius-Bode law, adding to a total of 10 if all hypothesized planets are counted. Let's take a look at the history of this system.
- The initial detection of planet candidates at Tau Ceti was made by Tuomi et al. in 2012. They detected five planet candidates via the radial velocity method, designated Tau Ceti b, c, d, e & f.
- Feng et al. 2017 detect four planets at Tau Ceti, two of which are the previously detected e & f, and two of which are new planets, designated g & h. They detect the radial velocity signal corresponding to planet d, but are unable to confirm it, and they characterize b & c as false positive detections.
- Kervella et al. 2019 detect a candidate long-period planet at Tau Ceti via Gaia astrometry.
- Coffinet et al. 2019 fail to detect any of the previous radial-velocity candidates except for planet h.
- Dietrich & Apai 2020, assuming the existence of Tau Ceti e, f, g & h, predict four additional planets in the Tau Ceti system. Three of these correspond to the b, c & d candidates, bringing b & c back to (dubious) candidate status rather than outright disproven. The fourth predicted planet would orbit between e & f, within the habitable zone, although it's important to keep in mind that there has been no detection of this hypothetical planet. Near the end of the paper, almost as an afterthought, they predict a fifth new planet based on the assumption that all previous predicted planets exist.
Additionally, the masses of both Tau Ceti e & f are estimated at around 3.9 Earth masses, which, given the limitations of the radial velocity method, are actually inclination-dependent lower limits (M*sin(i)). We can estimate the true masses of these planets by assuming their orbits to be coplanar with the Tau Ceti system's debris disk, which has an inclination of 35 degrees. This results in masses of around 6.8 Earth masses, in which case these planets are significantly more likely to be mini-Neptunes rather than terrestrial planets.
Given all this, Tau Ceti e & f are unlikely to be habitable. The hypothesized planets(s) orbiting between Tau Ceti e & f might be better candidates for habitability if they exist, but again, there has been no detection of these hypothetical planet(s).
Some text on planetary habitability will be added here in the future. Note that Eric Nelson's post on this topic below is based on the "Rare Earth" hypothesis, and it's not universally agreed that all of those factors are necessary for a planet to be habitable.
