The height of maninav is determined by the font-size and padding of what is inside it by ja-sosdmenu.css (line 81)
#ja-cssmenu li a {
background:transparent url(../images/mainnav-sep.gif) repeat-y scroll right bottom;
color:#C8C8C8;
display:block;
font-size:135%;
font-weight:bold;
line-height:normal;
padding:10px 30px;
text-decoration:none;
text-transform:uppercase;
}
<blockquote>when i open the mainwrap-bg.gif it shows me that this picture is 2000×1 pix, is that right? If i layout a new gif with this size the layout of the menu is wrong see below.</blockquote>
Yes that size is ok. You are not changing the layout you are changing the background.