Another useful site to get free PBR textures from is freepbr.com with texture maps for 150+ materials.
The best thing about this site is all of the textures are completely free with no watermarks, resolution limits, daily download limits or any other limitations. You can download textures freely as individual zips, with all the textures for a single material in a zip, or you can pay $5.00 for access to 6 large zips (roughly 500-800MB each) containing all the textures for all the materials.
Most texture resolutions are 2048x2048 and/or 4096x4096 but some are odd sizes (2685x2685 for example) that are not powers of 2 and thus can't have mip maps.
Quality is very much a mixed bag, some are pretty good, others really aren't very good at all. Hand adjustment through liberal use of lerp, power and multiply nodes in your materials can make many of the not so good ones look ok too.