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
The page definition as a string. The string is in format WxH+x+y. This method is available if Imagick has been compiled against ImageMagick version 6.3.6 or newer.
page
The page definition. For example 7168x5147+0+0
Returns true on success.