The sixth planet in the Hoth system was a habitable world used by the Rebel Alliance for a major base to replace the one exposed at Yavin. The fact that it is the sixth world implies that either the Hoth sun is more luminous than ours, or the inner planets have narrowly spaced orbits.
An upper limit can be placed on the size of Hoth 6. When General Veers' walker came close enough to the rebel power generator for it to appear over the horizon it was at a distance of "one-seven-decimal-two-eight". Considering the visual evidence and the tendency to use metric units in STAR WARS, this must mean 17.28km. Since the AT-AT head height is about 18m, if we assume that the walker was on flat ground then the radius of the planet is R < 8.3x106m. Of course, the walker really had the advantage of being on a ridge, so this is an upper limit rather than a direct calculation of the planet's radius.
A benefit of knowing the radius of the planet and the altitude of the rebel base (hence determining the distance to the local horizon) would be the ability to calculate a lower limit estimate on the area protected by the rebels' energy shield. Unless it penetrates the ground, the shield must extend at least to the horizon, otherwise Imperial ships could simply fire under the edges. According to Essential Guide to Weapons & Technology, the Hoth shield covered a region 50km in diameter meaning that it either wraps past the horizon, had a single generator placed at an exceptionally high altitude, sat on a planet somewhat larger than Earth, or else there were several nodes of generation up to and beyond the base horizon.
The surface of Hoth 6 is largely shrouded with cloud. About half of the surface area is visible from space. Half of the regions seen to date are oceans; the remainder appears to be covered with ice. Essential Guide to Moons & Planets suggests that the oceans are equatorial, and STAR WARS The Visual Dictionary hints that there may be some tundra in the equatorial regions as well, but it remains unseen in The Empire Strikes Back. The absence or scarcity of land plants is not cause for concern about the composition of the atmosphere; oceanic plankton is more than sufficient to maintain global oxygen levels and serve as the base of the food chain. It seems likely that much of Hoth's life may be concentrated in the seas and near the shores.
According to General Rieekan, the system had a great amount of "meteor activity". This probably means that there are a lot of asteroids crossing the planet's orbit, and it may have something to do with the presence of the Hoth asteroid field described in The Illustrated STAR WARS Universe. (This may have something to do with the dense and vigorous asteroid field seen in The Empire Strikes Back, but the location of that field is controversial.) Minor impact events are commonplace, like the one Luke Skywalker thought he would investigate. If small impacts are common, large impacts must be frequent on geological timescales. The planet would suffer the "nuclear winter" effect often, although not necessarily within recent history. The transient nuclear winter effects from modest and major impacts may cool the planet to the point where ice formation brightens the surface to the point where enough sunlight is reflected to sustain a perpetual ice age.
Several of the moons of Hoth have been photographed in front of the planet's disk, which places upper limits on their sizes and lower limits on the dimensions of their orbits. Three available images are usful for this kind of analysis. The first one is a distant portrait of the planet and its moons without any foreground objects. The second useful image is the view of Hoth during the arrival of Lord Vader's taskforce. The third image is a closer view of the planet and moons, near conjunction, during Luke Skywalker's departure.