Find Entries with Multiple Matches in a Single Field
The main problem is your URL isn't formatted correctly. You need brackets after each 'size' parameter to make an array of those values. Otherwise, it only picks up the last value.

http://domain.com/search/results?size[]=small&amp;size[]=medium

Then you can do something like this:

{% set sizeQuery = craft.request.getParam('size') %}

{% set params = {section: 'shorts', limit: null} %}

{% if sizeQuery %}
  {% set params = params|merge({'size':sizeQuery}) %}
{% endif %}


Also, you can use the dump() function to test what craft.request.getParam('size') is finding in your URL.

{{ dump(sizeQuery) }}


This URL: http://domain.com/search/results?size=small&amp;size=medium

... returns string(5) "medium".

But this URL: http://domain.com/search/results?size[]=small&amp;size[]=medium

... returns array(2) { [0]=&gt; string(5) "small" [1]=&gt; string(5) "medium" }.