From Forum Nokia Wiki

If there is a need for a custom password/secret editor, a custom control derived from CEikSecretEditor is required. Now CEikSecretEditor is the only editor that has not been derived from CEikEdwin. So it (And control derived from it, i.e. CAknNumericSecretEditor) does not have ConstructL variants to set size in pixels/characters. If a custom secret editor is required be sure to set its correct size using SetPosition and SetSize functions.

Also the password/secret control can never be created without a resource. So always have a dummy resource file for a Secret editor

RESOURCE SECRETED r_my_secret_editor
{
	num_letters = 10;
}

The num_letters can always be modified using the SetMaxLength function.

Also whenever the controls MinimumSize is called the default values will be returned. i.e. the size of the editor reported wont be the one you set using SetSize. It will be the size when the editor was created using

TResourceReader reader;
CCoeEnv::Static()->CreateResourceReaderLC(reader, R_MY_SECRET_EDITOR);
ConstructFromResourceL(reader);
CleanupStack::PopAndDestroy();

So always return the Rect what is set using SetSize function.