resize_to_limit

 resize_to_limit: [300, 300]

• shrinks larger dimension to match dimensions
• no effect on smaller images

resize_to_fit 

• shrinks larger dimension to match dimensions
• will scale up small images

reusable  variants 

class User < ApplicationRecord
  has_one_attached :avatar do |attachable|
    attachable.variant :thumb, resize_to_limit: [100, 100]
  end
end

<%= image_tag user.avatar.variant(:thumb) %>