You can do it in theme less file.
For example add this lines into end of modules/boonex/__your_theme_module__/data/template/system/css/menu.less
.bx-menu-floating-blocks-wide .sys-icons {
font-size: 2rem;
}
and clear LESS cashe after file changed
UPD
This example work for fonts. I haven't UNA installation with image icons for code check and write correct css classes for you
If you give me url with this page - I try to help you