Input size="20" not wider enough

[Incremental]

Hi,
I set F3.1rc2 on a J2.5 site.
My form is OK, but I would like to have wider input fields for my elements (Text, max length = 255).
As the CSS wide is OK (Firebug examination), my HTML is :

<input id="rencontres2014___Prenom" class="span1 fabrikinput inputbox text" type="text" value="" maxlength="255" size="20" name="rencontres2014___Prenom">

As 20 is not enough. How could I set size="40" to have wider fields ?
thanks

 

[troester]

Set "Bootstrap class" in element's "Formatting" part.

 

[Incremental]

Thanks troester
I haven't tried all bootstrap fields as I'm on J2.5
input-xlarge is fine for me