Как запретить изменение размера?

.header_menu>li {
  vertical-align: middle;
  display: inline-block;
  font-size: 17px;
  height: 40px;
}

.header_menu li>a {
  -webkit-transition: color .2s, background .2s, border .2s;
  -moz-transition: color .2s, background .2s, border .2s;
  -o-transition: color .2s, background .2s, border .2s;
  transition: color .2s, background .2s, border .2s;
}

.header_menu li:hover #sub_header_menu {
  display: block;
}

.header_menu li:hover {}

.header_menu li>a:hover {
  background-color: #FD4A29;
  color: #F0F0F8;
  -webkit-transition: color .2s, background .2s, border .2s;
  -moz-transition: color .2s, background .2s, border .2s;
  -o-transition: color .2s, background .2s, border .2s;
  transition: color .2s, background .2s, border .2s;
}

.header_menu a {
  display: block;
  border-right: 1px solid #F4F4FC;
  color: #777777;
  text-decoration: none;
  padding: 10px 15px;
}

#sub_header_menu {
  display: none;
}

#sub_header_menu a {
  border: 0;
}

#sub_header_menu li {
  list-style-type: none;
}

#sub_header_menu {
  border: 1px solid #F4F4FC;
  background: white;
}

<ul class="header_menu">
  <li><a href="#">4242342342</a></li>
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  <li>
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</ul>