The TikTok Video Cover size (1080×1920px)
TikTok is fully vertical and full-screen, so 1080x1920 pixels at 9:16 is the native size for video and the cover frame you choose to represent it. Uploading a cover at this exact ratio means it fills the screen and looks correct both while playing and as the thumbnail on your profile grid.
TikTok's interface is famously crowded on the sides and bottom: the right rail holds the like, comment, share, and profile buttons, while the bottom carries the username, caption, and audio ticker. Those overlays eat into the right side and roughly the bottom 300-plus pixels, so any cover text placed there gets hidden behind the UI.
There is also a profile-grid consideration. Your chosen cover becomes a slightly cropped thumbnail in your profile's grid, which is closer to a portrait tile than full 9:16, so keep the title and focal subject centered and high enough to survive that crop. A bold, centered hook that clears the right rail and bottom bar is what makes a cover work on TikTok.