Hi Massimo
Sorry it has taken a little while to get back to you, the support queues have been very busy.
The styles you are adding are not specific enough to override the existing styles set by, amongst other things, Bootstrap CSS.
Let me explain that first, and then explain why the styles you are adding won't work as you intend.
On my local test site I set up something similar, a View with 6 filter controls wrapped inside a Bootstrap grid of columns, so the markup looks like this:
<div class="row">
<div class="col-sm-2">
<div class="form-group">
<label>[wpml-string context="wpv-views"]Status[/wpml-string]</label>
[wpv-control-postmeta field="wpcf-status" type="select" url_param="wpv-wpcf-status"]
</div>
</div>
<div class="col-sm-2">
<div class="form-group">
<label>[wpml-string context="wpv-views"]Some text[/wpml-string]</label>
[wpv-control-postmeta field="wpcf-some-text" url_param="wpv-wpcf-some-text"]
</div>
</div>
<!--- continuing with more controls -->
</div>
Here is how the resulting markup actually looks:
<div class="row">
<div class="col-sm-2">
<div class="form-group">
<label>Status</label>
<select id="wpv_control_select_wpcf-status" name="wpv-wpcf-status" class="js-wpv-filter-trigger form-control">
<option value="" selected="selected"></option>
<option value="prospect">Prospect</option>
<option value="active">Active</option>
<option value="complete">Complete</option>
</select>
</div>
</div>
<div class="col-sm-2">
<div class="form-group">
<label>Some text</label>
<input type="text" id="wpv_control_textfield_wpv-wpcf-some-text" name="wpv-wpcf-some-text" value="" class="js-wpv-filter-trigger-delayed form-control">
</div>
</div>
<!--- more controls -->
</div>
Note that my select element has some styles, in particular .form-control.
Bootstrap applies some styles to .form-control.
A class selector is more specific (and so overrides) an element selector, so if I add a CSS rule for the select element it will be overridden by the .form-control style rules from Bootstrap.
So you need to make your style rules more specific, e.g.
select.form-control {
border: 2px solid red;
}
This will work. It will add a red border to the select dropdown.
Now, the styles you are adding for the select and inputs are to manipulate the the width and margins etc. You can do that, but it will only affect the width of the select dropdown within its grid column.
You are using a Bootstrap grid with 6 equal-width columns. In your browser console inspect the container div with class .col-sm-2 and you will see that is where the width and other settings are that you would need to modify.
You can add classes to your markup to individual wrapper divs and then add styles to overrule the width for that particular column, remembering that they must add up to 100%, so if you make one column wider you will have to make another smaller.
Or, stop using the Bootstrap grid and come up with your own CSS rules for how the filter controls are laid out, such as flexbox.