Unfortunately you're not going to be able to find larger sizes of the Venusaur / Charizard art unless you can find someone who scans the boxes in themselves and renders the scans (re-drawing over the parts covered by logos, etc.) or if you can find wallpapers that were released with the images and render them from there. I don't recall wallpapers being released with those on them, but then again I wasn't really looking for them back then.
As far as the Giratina is concerned, I believe all I've ever been able to find is larger versions on Platinum Version wallpapers. Going off the U.S. site, the only Giratina-related one features its other Sugimori art for Origin forme (not the box art he did of it). The largest I can find right now is pic related.
