Not the developer but hopefully this will help
You need to add
background-size: 100%;
under the code containing the social icons you wish to make larger then change the css value for example
.social_icons .social_logos li.fb a {
background: url('images/facebook.png') no-repeat 0 0;
background-size: 100%;
width: 25px;
height: 25px;
The only thing is the original images used by the developer are too small and blur when given a larger value therefore will either have to change the links to different images or replace the original images in your wp-content/themes/one-page/images to the images of your choice!
Also to accommodate the larger images you will need to change the height of the top bar.
This is under the .social_wrapper
This is trial and error depending how larger you have made your icons. a general rule is increase it first by the difference in size then fine tweak it.
Hope this helps. If you need any more help let me know