The install size for Overwatch on Xbox One X has been revealed coming in at an alright size with the enhanced option being available starting January 30, 2018 taking up a total of 21.00 GB on the Xbox Marketplace. The Xbox One Base size is roughly the same size as the general update was a minimal change. The install size was grabbed from the console with the game having been given a fairly average less than a 1GB update and it now showing up under "Xbox One X Enhanced Games" tab. 4k assets for games are automatically available to those on the Xbox One X.
There are additionally no mentions of Overwatch receiving HDR support on the console and it was excluded mention wise from the patch notes. Since it hasn't been added this long after release I wouldn't anticipate HDR being present in the future. Details are limited for this update essentially just being a boost to the resolution offering. It already ran incredibly smooth across the platforms and now it looks best visually on the Xbox One X. The vibrant colors of this shooter will certainly pop as the world and the various characters are rather bright. This install size will most likely change a fair bit while staying relatively small overall. It's hard to judge with the title since it receives constant improvements and tweaks yet Blizzard is amazing at keeping it compact with its install size.