Is there any way to resize a title clip to change the aspect ratio? I have been using the pan and zoom effect to put the title in a certain area in the video, and I want to be able to use all the vertical space.

I admit that I do not entirely understand what you mean by changing the aspect ratio. But you may achieve the desired effect by using any paint program to create a png graphic that uses all the space you wish.

You can make the font arbitrarily large. Wouldn't this will then make it fill the space?
You can resize it via the affine transition that is placed on the time line with it.
And yes you can change its aspect ratio via the affine transition if you check the distort check box and then adjust the width and height.