Increase thumbnail width / height

Fixes: GNOME/epiphany#605
3 jobs for <span class="ref-name">thumbnails</span> in 10 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #178442
flatpak

00:09:58

 
  Deploy
passed #178443
review

00:00:35

manual #178444
allowed to fail manual
stop_review