I'm going to go out on a limb and say that it might depend more on aspect ratio than screen size. I am going to assume by the dimensions you give (i.e. same width but 6" shorter) that your current TV is 4:3 aspect ratio. That is to say that when you watch standard def TV, it fills the whole screen but when you watch widescreen, you have black bars across the top and bottom.
If that is the case, then you will probably have a bigger image in widescreen since the image will fill the whole screen of the new TV. The trade off will be that 4:3 SD images will be smaller because you will have black bars on the right and left sides. It's hard to say without knowing which products you are comparing but if I'm reading you right, then your HD broadcast and 16:9 movie content will be larger and this will be an excellent move.
