If you are speaking about front-end, you can customize thumbnail using:

public class ImageData : MediaData
{
    /// <summary>
    /// Gets or sets the generated thumbnail for this media.
    /// </summary>
    [ImageDescriptor(Width = 48, Height = 48)]
    public override Blob Thumbnail
    {
        get { return base.Thumbnail; }
        set { base.Thumbnail = value; }
    }
}

more detail:

http://world.episerver.com/documentation/Items/Developers-Guide/Episerver-CMS/8/Content/Assets-and-media/Media-types-and-templates/

Aria

So if you need to change size within Media Library you may need to:

1. Inject custom CSS and override 48px X 40px
2. Override Thumbnail and change the width and height of pixel

Aria