I might be wrong with this, but couldn't you force dimensions of a container via a div tag around the container structure. I do this with panes all the time to strectch / shrink them to my needs. The main problem I see with this approach though is tons and tons of odd looking white space at the bottom of the container, cause the module is only going to take up X,y space. But the outlines / borders of the containers would remain the same size, if you set scrolling to true then html module should create a scroll bar to fit extra content instead of expanding past its boundaries. I haven't had to do this though and sure there would be other smaller difficulties to overcome. The only other option I can think of is to recode some of the core modules and force height / width limits from within the module. This could get messy though, cause your re-coding to fit one page might not work well on another, so then your having to create copies / variations of modules. Not a good approach. Are you just wanting the borders of the left / right side to be the same, or do you want all content in the left pane to remain even with the bottom of the content pane after the login? Or do you want to just even out your login so its centered a bit more down the left side?
There are a few "workaround" tricks I know to help fake positions of modules. For example while not perfect you can place a html/text module at the top of your left pane, press enter a few times. Set the module to not display a container. This will push the login module down the left pane. The only thing a user will ever know is if they drag their mouse in the "invisible" module they will see the little highlight bars for the lines you entered. (the not perfect part) However, its a simple fix to push a module down the page a bit.