I use .xinitrc in order to display two small RXVT windows when I start X. Here is what the bottom of my .xinitrc looks like:

#I added this for a second RXVT
rxvt -geometry 70x20-0+0 &

#Start the Window Manager
exec /usr/X11R6/bin/fluxbox &
rxvt -geometry 70X20+0+0

As you can see, I was able to size the RXVT windows using '-geometry.'

To my question.

I want to display a small Mozilla Firebird window, as well, when I start X. I can get the Mozilla Firebird window to display, but at near full screen only, when my .xinitrc looks like:

#I added this for a second RXVT
rxvt -geometry 70x20-0+0 &
exec /home/bdw/MozillaFirebird/MozillaFirebird &

#Start the Window Manager
exec /usr/X11R6/bin/fluxbox &
rxvt -geometry 70X20+0+0

I have been unable to resize the Mozilla Firebird window, a la the RXVT windows and could use some advice. The Mozilla Firebird displays when I start X at near full screen and I would like it to display in a small window in the middle of the screen (about 40X20).

I'm beginning to learn that most apps have a -help or --help option that can be very useful. I was able to resolve this silly problem by executing Mozilla Firebird in my .xinitrc like "exec MozillaFirebird/MozillaFirebird -height 200 -width 700 &." Now it lauches in the smaller window I wanted.

Yay!

"man app-name" or "info app-name" helps a lot too.
There are also configuration files (dot files, rc files, whatever you'd like to call them) for most applications, and some of them allow you to specify window geometry that way.
Many of these dot files are automatically created in your home directory the first time you launch an application...others are available to be copied from /etc or from the original application archive. Still others sadly don't exist at all. It's my belief that every application should have an rc of some kind.