Front Page Thumbnails

Trying to figure out how to disable the crop of the front page thumbnail to show both horizontal and vertical oriented images, depending on the feature image format.

I just want the width to be fixed with unlimited height, keeping the proportions constrained. Already tried with add_image_size( ‘teaser-thumb’, 332 ) and some other combinations but I’m obviously doing something wrong because it doesn’t work.

Thanks in advance,
Lara