App Store

• App name: 30 characters
• Subtitle: 30 characters
• Keywords: 100 bytes, comma separated, no spaces needed
• Promotional text: 170 characters, can change without a new version
• Description: 4,000 characters

Source: App Store Connect help.

Google Play

• App name: 30 characters
• Short description: 80 characters
• Full description: 4,000 characters

Source: Play Console help.