Sometimes it can be unclear how to format Attachment_Fu or Paper Clip resizing of images.
I came across this geometry guide that should help be an quick reference.
% = resize to a percentage
! = absolute resize
@ = maximun area
< = resize if both w & h do not match
> = resize if w or h does not match
"x300" resizes to a height of 300
"x300>" resizes to a height of 300 if the height is greater than 300
"150x100>" resizes to 100 height if height is greater than 100 and resizes the width appropriately
"50%" resizes the image 50% of the width and height