In application.properties file write this:
# Max file size.
spring.http.multipart.max-file-size=1Mb
# Max request size.
spring.http.multipart.max-request-size=10Mb
Adjust size according to your need.
Update
Note: As of Spring Boot 2, however you can now do
# Max file size.
spring.servlet.multipart.max-file-size=1MB
# Max request size.
spring.servlet.multipart.max-request-size=10MB