Android security patches normally start rolling out to the first few devices to get them, like Nexus and Pixel phones, pretty early on in the month, and this one should be no exception, with indications that it will be hitting the streets on January 3rd. To be precise, these updates normally go out on the first Monday of a given month. With New Year's Day having been the day before that in this case, however, it's understandable that Verizon's support page for the Pixel and Pixel XL shows the update as being pushed out until the third of the month, with that day being a Tuesday. Since Verizon's Pixel models get their updates straight from Google, all Pixel variants around the world will likely see similar timing for their updates to roll out, and then Nexus devices will start to see the update a little bit later on.
For Verizon Pixel users, the update will raise your software version to NMF26U. According to Verizon's documentation, you'll get the latest Android security patch, and nothing else. Verizon's documentation on their web site does not include the size that users can expect the download to be, but it should be smaller than the December update; that one had a security patch, an update to Android 7.1.1 Nougat, and a good amount of miscellaneous fixes on board, contributing to a pretty substantial file size.
The new security update should bring about the usual number of fixes and vulnerability patches to the table, perhaps even a little less due to Android 7.1.1 having just rolled out; this means that white hat hackers and malware makers alike haven't had much time to tinker with it and find things that Google will need to patch just yet. That isn't to say both camps don't have their engines running, of course, and it'll be a race against time to see how it plays out. Since this is just a security patch, users who have supported devices shouldn't expect any large files to be sent over-the-air, but as always it's good practice to download the software while connected to Wi-Fi instead of the cellular network if you're worried about your amount of 4G LTE data.