How to change the dropdown boxes dimensions ?

Please read the FAQ,

How can I styling the search form?
You can simply refer the themes/default.css within the plugin folder and alter it or override it at your theme’s CSS file.

And since where here,do you know how I can limit the number of items by page ?

You can configure the setting in the form setting page, at the Results And Others->Results per Page

The css is out of this support scope. You should seek help by google or other resources.
However, in the css file you can see the comment of each of the element and what you can do with it.
Eg:

/*the form div*/
/*you can defined the width of the entire form*/
#uwpqsf_id{border:1px solid #ccc;width:200px}

/*each of the filter class, you can set the width to 50%*/
.uwpqsf_class{width:50%;margin-bottom:5px;clear:both}

To set the form in the center of the page, you will need to google it, there are plenty of resources/tutorial out there.