You can change the tool window's DockedSize property before you programmatically call the DockTo method. That will set the "desired" size and depending on the docking situation and space available, it will try to dock at that size.
However, changing it while docked will not do anything. The reason is that tool windows might be nested down many levels of dock containers deep so programmatically resizing a tool window can be a very complex operation. We do have it on our TODO list to try and implement in the future though.