I did that using memory editor (cheat engine). It's 400 width and 475 (i think) height.
Search for 400, you end up with 800 - 1000 addresses and then you have to manually find it. you can either debug and write a script (not sure if skype likes it) or you can do pointer scan. either way, you would have to run a small "trainer" to apply this each time skype starts, i tried reshacker on skype, but skype somehow knows it has been tampered with and doesn't start.
Maybe hex edit on skype.exe would do, but i don't know how to convert memory location to a physical .exe location.
Edit: pointer scan works on ALL convo windows'. If i do pointer scan and freeze that point to like 200, then any window i open will have resize limit of 200.
I'm sure there's easier way using a debugger, like debug .exe using olly and then write those changes into .exe, but i'm not so good with .exe debugging.