The normal aspect ratio of a widescreen TV is 16:9 (the ratio is 16/9=1.778). Currently your photo ratio is 6/4.5=1.334 (which is the non-widescreen TV aspect ratio of 4:3). You only need to crop the height. 6/1.778=3.375 so the new size is 6"x3.375".
However, if you're not printing the pictures, measuring in inches is pretty much irrelevant. Ideally you want to work in pixels as opposed to inches, so if your photos are 1024pixels by 768pixels (4:3) your new size would be 1024 by 576 (1024/1.778=576).
Incidentally, 5x3 (10x6, etc) is the closest standard picture frame size to widescreen (5/3=1.667).