attachment_fu width

Does anyone know how to resize images according to a specific width
using attachment_fu? More specifically, I am trying to resize images
so that they retain their ratio but all have the same width. Thanks.

:thumbnails => { :large => '350x350>'} is the syntax I used and they
retained their ratios.

right, but i think this makes the images within the 350x350 range with
retained aspect ratio, but i want to make every image have the same
width and then make the height according to the ratio. does this make
any sense?