Autoscale

In "Galleria -> Edit Option Set" add responsive:true and set width to auto or delete it
In your CSS set field-wrappers width to 80% and .galleria-container to 100%

And you probably need to clear Drupal and Browser caches to see changes

hm well, i got

Error: Fatal error: Could not extract a stage height from the CSS. Traced height: 0px.

ok well NOT WORKING

.galleria-container {
  width: 100%;
  height: 100%;
}

.field-name-field-galleriebild {
  width: 80% !important;
}

changes nothing, galleria does not appear, meaning cant get size

If you set the galleria height option to a number below 2 it will use it as a ratio instead of a height in pixels. You don't need to supply a width in your optionset.

See: http://galleria.io/docs/options/height/

Then just set a % width in your css and galleria will adapt and scale. It works exceptionally well.