The two features can be called from the 'extra'-menu (see the screenshot) or alternatively using the shortcuts [ctrl]+[alt]+[P] to fit the page or [ctrl]+[alt]+[O] to fit to an object in the page.
The automatic window resize on page-load can be disabled in the settings, you can then call the features only when you need them. To prevent misuse and/or misbehaviour, maximum and minimum sizes can be configured in the settings.
Please report your experience and/or feedback. This is my very first extension, so please be patient with me.
The extension is licensed under GPL 3.0, please respect this licence.
EDIT from 2013-12-18: Sorry that this addon is currently quite buggy on some pages. I will try to fix that as soon as i can. Currently i am a bit short on time but that will be done someday as it annoys me the same as it probably annoys you. Sorry!