If you want images the same size, use the crop tool, set the H & W to specific dimensions and (for the web) use 72 dpi. Then whatever you crop, will be that size. Of course, the default for the size setting is inches (at least in v6 that I'm still using). If you want to set a dimension in pixels, type the number of pixels followed by "px" in both H and W, as in "500 px" x "350 px". That's if you want them all the same proportion. If you only care about the height, I think you enter that info in one box and leave the other blank (or zero?). What usually do though, is crop the image first with no dimensions, then use Image/ImageSize and set the height in pixels (500) and the width goes where it goes.
Then you can use the web tool (which I don't use, and I'm sure its different in CS) to automatically create thumbnails keeping the same height, but either different or the same width ratio, if you created them that way. (I currently use ThumbsPlus for all my auto web page making and it does the same thing).
Andy Pearlman Studio