Greetings all!
Where should i do to modify menu tab width? (menu i mean pages over header image)

Thanks in advance

Menu tab width is determined by the text inside it unless you override with custom CSS.

Thanks, where i should go to modify the header menu class and add width:200px? (example)

thanks in advance

You would add something like the following to ATO>Add HTML/CSS Inserts>CSS Inserts.

div#menu1 li {
width: 200px;
}