Simple responsive css/html dropdown list (mobile and desktop)

[Ensayos]

I have a Spanish based language site: http://www.ensayostube.com/ with informations about different topics.

I want to implement a simple responsive css/html dropdown list (mobile and desktop) for my site on pages like this on the right side:
http://www.ensayostube.com/documentos/ingenieria/electronica/3.php
 of the site categories :

Biología
Ciencia
Física
Francés
Gramática
Inglés
Literatura
Matemáticas
Ordenador
Pedagogía
Química

i want to be on dropdownlist.

But the dropdown shold be SMALL on MOBILE and bigger on desktop or tablets

How can i do that?

I need some html/css code suggestions, to make this.

Thanks all

[Dess | Tech Support Engineer, Principal]

Hello, Ensayos, 

Thank you for writing.  

I would like to note that this forum is related to the Telerik UI for WinForms suite. Could you please specify what is the exact product that you are using? In case you don't use Telerik UI for WinForms, feel free to post your questions in the relevant forum: http://www.telerik.com/forums

Thank you for your understanding.

Regards,
Dess
Progress Telerik