Hi,
There is a bug when using conjointly these two features. Here are the relevant portions of configuration :
;;make ibuffer resize itself to its buffer size
(setq ibuffer-use-other-window t)
(setq ibuffer-default-shrink-to-minimum-size t)

No specific configuration for erc, just enabling tracker is enough.
Then, run erc, and, in any erc buffer, call ibuffer. The screen briefly flashes as an ERC window opens then disappears, and the ibuffer window is reduced to a much smaller size than what it would be outside ERC. I think the issue is erc's hook to window-configuration-change somehow messing with ibuffer, but I'm not experienced enough to find where the bug is.

Antoine Levitt