Which version of ICQ are you referring to? I was running ICQ on a 640x480 monitor back then and i remember it having a very compact window[0] which i always had it visible.
It's been so long I really can't recall anymore. (Edit: but just like you, I do remember that I had the ICQ window visible at pretty much all times, and I could comfortably fit an Emacs window, the contact window a chat window on my desktop's 1024x768 screen.)
That being said, the window in that screenshot is about 200 x 320 px. It would have taken about a third of the horizontal space of the screen, and about 2/3rds of the vertical space of a 640x480 screen. It was certainly usable -- way, way more usable than Skype on a full HD monitor today -- but lots of stuff was claustrophobic in 640x480.
(Then again, most modern apps are practically unusable if you resize their window to 640x480...)
Sadly Skype and similar programs tend to take more physical space on my monitor than ICQ (and MSN Messenger, especially the original versions like the one included in Windows XP) ever did :-/
The latest version of ICQ was huge (think Windows XP skin), at this point I started using the QIP alternative client which was even more compact than the first version of the original client.
When everybody started using Miranda, Pidgin file transfer pretty much broke due to client incompatibility. That was a sad moment, they had a nice product but stopped listening to the users.
That being said, the window in that screenshot is about 200 x 320 px. It would have taken about a third of the horizontal space of the screen, and about 2/3rds of the vertical space of a 640x480 screen. It was certainly usable -- way, way more usable than Skype on a full HD monitor today -- but lots of stuff was claustrophobic in 640x480.
(Then again, most modern apps are practically unusable if you resize their window to 640x480...)