There are a few things to remember when doing a batch image resize operation.
Firstly, the same restrictions apply as when you are doing it manually. If you make an image much bigger than it was originally, you will notice a quality degradation. Generally there's no problem making images smaller, but a loss of quality when blowing up images is unavoidable, since the image does not gain any new data, it just stretches what's there!
Secondly, and especially if you are using the 'Constrain Proportions' option, if you have a mix of portrait and landscape images, be careful! If you specify the width for example, and then set Photoshop resizing, the portrait images will end up larger than the landscape images. It might be an idea to separate out the two types and batch resize them in two separate groups.
Lastly, when you hit that 'Ok' button, be prepared to stop using your computer while Photoshop performs the batch resize. To re-dimension the images Photoshop must open each file, and this can be quite a big operation.
You can do other things while Photoshop performs a batch image resize, but I've found that if there are a lot of images, especially if they are large files, then it can cause Photoshop to freeze or crash, so it's best just to be patient where possible.
Once you have selected a set of images to batch resize using one of the methods above, you need to refer to the third frame in the box, titled 'Image Size' First tick the box that reads 'Resize Images'. This will enable the frame. You now need to tell Photoshop what dimensions and resolution you would like the new images to have.
These options work just like those found in the 'Image Size' dialog box available from the 'Image' menu.
Generally I like to specify the width and height in pixels. If you wish to do the same, make sure "Pixels" is selected in both drop down boxes. Then you can enter your values.
If the 'Constrain Proportions' check box is not ticked, you will have to specify both dimensions. This will resize all images according to those dimensions, even if this distorts the image.
If 'Constrain Proportions' is ticked, then you can only specify one dimension. Using this method, you specify either a width or a height, and each image is scaled so that it matches the dimension you specified, and its other dimension is such that the image is still in proportion.
Finally you need to specify a resolution. This is the only noticeable difference from the 'Image Size' dialog box. Here you can only pick from a number of preset resolution values, instead of being able to specify any resolution you like.
Remember that the resolution is stated in Pixels Per Inch (ppi) not Dots Per Inch (dpi).