Slack custom emojis must be 128x128 pixels or smaller. Images are automatically resized if larger, but pre-sizing to exactly 128x128 gives you the best control over how it looks.
What is the file size limit for Slack emojis?
Slack custom emojis must be under 256KB. Use JPEG for photos or PNG for graphics. If your image is too large, compress it after resizing to 128x128.
