Changing a custom header size

probably yes;
if you have a self-hosted blog, and use the ww.wp.xz.cn software.

find the theme file where the custom header code is located – could be functions.php, or a separate file – and try and edit these numbers; there are likely to be many more places where these numbers need adapting, for instance in some style sheets.

what is your theme?
download link?

someone might have a look at it, and might be able to give you more information.