Sticky header for wordpress changes size and logo size as I scroll down

you can modify style in custom css code in theme options or install plugin custom css or modify the main css files

1.- for logo size in sticky header add this code in custom css or modify file in
/wp-content/themes/themify-ultra/styles/header-horizontal.css line#21 change height to 60px from 30px

.fixed-header-on #site-logo img {
  height: 60px;
  width: auto;
  transition: all .3s;
}

2.- for sticky header background color add the below code in custom css or modify in:

.transparent-header #headerwrap.fixed-header {
  background-color: transparent;
}