All 202 episodes are included in the box set. All have been digitally remastered and re-formatted for widescreen, taken from the original film stock. Fox have been working on this for at least two years, quietly releasing HD versions of episodes on reruns, Netflix and to foreign markets, apparently testing audience reception and reactions. This is in contrast to Star Trek: The Next Generation's slow, labourious release on Blu-Ray over more than three years.
The previous HD versions of the episodes released to television had some issues, most notably that some tracking and establishing shots of famous locations such as Washington D.C. were sourced from public archives and thus weren't re-mastered for HD. It's unclear if these have been fixed. There were also issues with some effects shots being upscaled rather than re-rendered from scratch. The X-Files, being more about mood and tone, has very few major effects sequences, but whilst that means such cost-cutting measures aren't hugely noticeable, they also would have been quite cheap to properly fix. It's unclear if the Blu-Ray editions will resolve these issues as well.
Some fans may also be a little bit upset that the two X-Files theatrical movies are not included, and neither is the show's spin-off, The Lone Gunmen, or the companion series Millennium, which is set in the same universe but only tangentially related. Given those show's low ratings and relative obscurity, a "mega-complete" franchise edition is probably the only hope either have for a HD remastering, but it's not happening this time around.
There will be a ton of special features, a mixture of new material and old stuff from the existing DVD sets.
No final price has been set for the box set, but I suspect in the UK it will be in the £150-£200 ball park. For the more budget-conscious fan, the seasons are also being released individually on the same day.
The new six-part X-Files mini-series debuts in the United States on 26 January 2016.