Compact Trello userstyle. If you have the Stylus extension, you can download the JSON and use "Import styles".

compact-trello.css

@-moz-document domain("trello.com") {
.card-label.mod-card-front {
    width: 8px;
	  height: 32px;
}
.list-card-labels {
    margin-right: 6px;
    margin-top: 2px;
    float: left;
}
.list-card-members .member, .list-card-members .member-avatar {
    height: 18px;
    width: 18px;
}
.list-card-members .member-initials {
    font-size: 8pt;
    line-height: 18px;
}
.list-card-title {
    clear: none;
}
}

compact-trello.json

[
	{
		"enabled": true,
		"updateUrl": null,
		"md5Url": null,
		"url": null,
		"originalMd5": null,
		"name": "Compact Trello",
		"sections": [
			{
				"code": ".card-label.mod-card-front {\n    width: 8px;\n\theight: 32px;\n}\n.list-card-labels {\n    margin-right: 6px;\n    margin-top: 2px;\n    float: left;\n}\n.list-card-members .member, .list-card-members .member-avatar {\n    height: 18px;\n    width: 18px;\n}\n.list-card-members .member-initials {\n    font-size: 8pt;\n    line-height: 18px;\n}\n.list-card-title {\n    clear: none;\n}",
				"urls": [],
				"urlPrefixes": [],
				"domains": [
					"trello.com"
				],
				"regexps": []
			}
		],
		"id": 1
	}
]