This seems to be no longer working, the setting in the manifest.yml is ignored and the panel is always rendered at 262px height. Can you please confirm if it’s possible to get a bigger panel than this size.
Similar to the other thread about jiraServiceManagement:portalSubheader, I recommend opening a ticket with developer support so we can get a proper bug filed. Would one of you be so kind as to report back here when you have the resulting issue key so future developers can watch, vote, and comment on it?