Limit file size for "image url" field
You should be able to implement a hook via PHP to validate the profile when it is submitted: https://docs.civicrm.org/dev/en/latest/hooks/hook_civicrm_processProfile/.

Someone else may want to chime in. I'm not sure if you can use form hooks with profiles. Otherwise you may check this: https://docs.civicrm.org/dev/en/latest/hooks/hook_civicrm_validateForm/

You can then use PHP to validate the file size:
Check this thread (https://stackoverflow.com/questions/9153224/how-to-limit-file-upload-type-file-size-in-php).