An alternative would be too select all objects from all frames at once, and then resize them all together... but i can't seem to select across multiple frames either (even when "display all" is selected)
Provided you have 'All Frames Visible' & 'Edit All Frames' it's a simple task to resize all objects within the animation. Then just export the animation as a gif choosing 'Page Content' as the export option.

EDIT: Where is the 'Display All' option?