I saw in chunk generator example plugin, that it returns bloks array with the height of 128 pixels. Can I return more blocks to make the map height bigger? Does server/client support it?
The server doesn't really support it either. The 128 cap is pretty hard-coded into the file save format. You'd have to do some significant modding to change it.
As a general rule, Bukkit does not add or modify any game features. It just adds the plugin framework. So, the answer is no, they can't.