Use Large Thumbnails

I would like to have large thumbnails i.e. 300 x 300!
I have used the code below in the CSS in plugin but thumbnails remain small!

.document-icon-wrapper .document-icon{
border-right: 1px solid #37824A;
border-bottom: 1px solid #37824A;
}
div.document-icon a{
font-size: 14px;
line-height: 16px;
}
div.document-icon img {
max-width: 300px !important;
max-height: 300px !important;
}
div.descriptions.document-icon img {
max-width: 300px !important;
max-height: 300px !important;
}

Any help greatly appreciated.

http://mrmook.com/demo1/gardinol/

By the way GREAAAAAAAAAAAAT Plugin 🙂

https://ww.wp.xz.cn/plugins/document-gallery/