Hello

It can be done via CSS only. If you are using default FontAwesome icons then you may add something like the following in Studio -> Decorous -> Styles -> Custom Styles:

#bx-sliding-menu-sys_site.bx-sliding-smenu-main .bx-menu-object-sys_site li .sys-icon {
    line-height: 3rem;
    font-size: 2.2rem;
}
#bx-sliding-menu-sys_site.bx-sliding-smenu-main .bx-menu-object-sys_site li .sys-icon {
    width: 3rem;
    height: 3rem;
}

You are welcome to use your own numbers.

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Best Regards
Anton