The entire map is reeeeaaaaaaally big and open and it's all nothing but huge corridors, with massive sightlines and no cover whatsoever, linking into eachother with one gigantic room in the middle. Also everything seems a bit dark.
Of course, this being your first map it won't really be very good, and that's just the way it works (my first map was a heathen to map makers everywhere). I suggest you scrap this one and start over again, studying official maps and incorporating Valve's/community map makers' techniques into your own map. There are plenty of tutorials on the site to help you make high-quality maps, as well as VMFs of official maps included in the Source SDK which you can study, taking into account things such as area sizes and flanks and whatnot. And who knows, maybe in the future you'll make the next big community map!