You should to use standard video resolutions, so 16x9 aspect ratio, which is 1920x1080 for HD or 3840x2160 for 4K. Some sites accept cinema 4K which is slightly wider at 4096x2160, but I would stick with 16x9. Strange aspect ratios probably won't be accepted any most stock sites. If you want to create vertical animations, you need to turn them sideways so you're still at 16x9. Also, Adobe stock doesn't accept vertical videos, so I would just try to create everything as horizontals unless you have a really good reason for a vertical video.