yoursite_extension.rb

# Samples for formatting additional Product fields in Spree Admin 
# SelectField: adds a select box with values 1..20
# WideField:   adds a standard text box, but with a specified size
# HugeField:   adds a text area with a specified number of columns

Variant.additional_fields += [
        { :name => 'SelectField',
          :only => [:product],
          :use  => 'select',
          :value => Proc.new{ (1..20).collect{|i| [i,i]} }
        },
        { :name => 'WideField',
          :only => [:product],
          :options => { :size => '180'}
        },
        { :name => 'HugeField',
          :only => [:product],
          :use  => 'text_area',
          :options => { :cols => '100'}
        }
]