In the Stackoverflow article you mentioned, the "True"/"False" values refer to packing, I think.
I think you're right... changing those didn't make any difference to me, but that's the closest thing I've been able to find.
You can also set a "width request" and "height request". I'm using Glade, so I usually don't look up the commands. Also, I'm using Python, not sure if I could help if you're using C or something else.
I am not using Glade (I know it'd be easier, but I'm trying to learn pygtk, so I don't want to take the easy way just yet), and I am using Python, so if you have any tips, please let me know. I tried googling with the terms "height request," but I keep getting the same things. If I remember, I'll try to extract a meaningful chunk of code tonight.