Yes, there is actually a way to open the game in windowed mode.
With Steam open, in your library find the “Black Mesa” entry and right click, find properties and select that. There is a button labeled “SET LAUNCH OPTIONS…” where once you’ve clicked that, a small window with a text field will open. Type exactly the following:
-window
Click OK, and so forth. I’m not sure the resolution commands, but this will at least get you started in the right direction.
(Haha. I just realized, your first post on the forums is the question and my first post is the answer.) Enjoy the game! I know I am.