In WordPress the default maximum file size to upload a file is 2MB.This is not an error but this makes uncomfort to beginners.They installs the plugins to overcome this issue.If you install plugins for small small issues then your website becomes fatty(becomes more weight or will load slowly) and your website may get hacked by the hackers or may stucks with spam.
To increase image size you do just add below code to your php.ini file.If you are not having this file in your public_html directory,No Worries,create php.ini file and add below code.
upload_max_filesize = 30M
post_max_size = 30M
Depending on your hosting,you can add the above code in your .htaccess file.If you do any of the above work on your website then you will be come out from that max file size to upload issue.