Short answer: 1080 × 1920 px, ratio 9:16.
Safe zone
TikTok places the caption and username at the bottom, and the like, comment and share buttons on the right. Keep important text in the centre — roughly the dashed area above.
Profile grid
Covers in your profile grid are cropped to about 3:4 around the centre. A title in the middle stays visible in both the video and the grid.
Other formats
Photo posts (carousels) also work best at 1080 × 1920; landscape 16:9 videos are shown with bars above and below.