This function seems to do the same thing as photoshop feture convas size, you set size of convas and position of your image's top left corner (x,y) inside that convas.
(PECL imagick 2 >= 2.3.0, PECL imagick 3)
Imagick::resetImagePage — Reset image page
$page): boolThe page definition as a string. The string is in format WxH+x+y. Este método estará disponível se a extensão Imagick tiver sido compilada com a ImageMagick versão 6.3.6 ou superior.
page
The page definition. For example 7168x5147+0+0
Retorna true em caso de sucesso.