What is the best canvas size for the android app design?

Depends. Are you talking about UI design using photoshop, or desing based on xml and java through an IDE (eg. Android studio)?Generally, initial stages of an app UI design is done over photoshop. A preffered size would be 1080x1920 for portrait mode, or 1920x1080 pixels for landscape mode. Sometimes you might want to design for recent phones with 2k resolution screens, use 2560 x 1440 pixels.