Non-model field is not validated in nested form - ruby-on-rails-3

Question

I'm trying to add a validation of 'size' property for 'Creative' model. 'Creative' model is one-to-one related to the 'Ad', and it's field are displayed as a nested form in the 'Ads' form(code provided below).
The problem is, that when I'm actually saving the form, size is not validated — neither with format validator, nor with :presence validator(if I add one). At the same time, :filter property is validated.
Size is a non-model property, added with attr_acessor method and custom setters and getters.
Looking for a way to fix this code or any different solution to add a validation for non-model nested property will be much appreciated.
Controllers:
class Creative < ActiveRecord::Base
attr_accessible :size, :filter
belongs_to :ad
attr_accessor :size
validates :size, :format => { :with => /^\d+x\d+$/, :message => "Incorrect size param. Should be [width]x[height], e.g. 320x50" }
validates :filters, :presence => true
def size
"#{self.width}x#{self.height}"
end
def size=(size)
#width, #height = size.split("x").map(&:to_i)
end
end
class Ad < ActiveRecord::Base
attr_accessible :creative_attributes
has_one :creative
accepts_nested_attributes_for :creative
validates_associated :creative
end
class Campaign < ActiveRecord::Base
has_many :ads
end
Form:
<%= simple_form_for [:rtb, #campaign, #ad], :html => { :class => 'form-horizontal' } do |f| %>
<% if #ad.errors.any? %>
<div id="error_explanation" class="alert alert-error">
<h2><%= pluralize(#ad.errors.count, "error") %> prohibited this ad from being saved:</h2>
<ul>
<% #ad.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
</ul>
</div>
<% end %>
<%= f.input :name %>
<%= f.simple_fields_for :creative do |creative_f| %>
<fieldset>
<legend>Creative Settings</legend>
<%= creative_f.input :size %>
<%= creative_f.input :filters %>
</fieldset>
<% end %>
<%= f.button :submit, :class => 'btn-primary' %>
<% end %>