I guess I should add:

"export all layers as images" by using FILE/SCRIPT/Export Layers to Files

"load those images into a stack" by using FILE/SCRIPTS/Load Files into Stack

Well, I probably spoke too soon. Exporting layers to files will save each file at the same size as the canvas and not just opaque pixels... let me think... because I've done this before.

I don't want all the images to be the same size

Oh, well then you will have to manually resize each layer. You can click on a layer, hit Ctrl+T for Transform, then up in the width and height area, click on the lock to maintain proportions, then for the width (or height) type in the value like 3in for 3 inches... you only have to do this for either the width or height.Then hit ENTER to accept transform. But, doing this may leave the edges of your images anti-aliased.

Going to go wipe the egg of my face now.

Just add some bacon and you will be fine.

If you wanted to resize selected layers you could use this script...

That might be a faster solution than manually, but like I said before about your resized edges may have anti-aliasing... if you notice this and/or if it becomes an issue, you can change your Image Interpolation settings under Preferences/General/Image Interpolation and choose Nearest Neighbor. Now, if you notice the pixels get too chunky after resizing, change the interpolation to give you the best results for resizing.

You could use a configurator panel and put the code under a button.

Unfortunately if you are using Windows 7 and Photoshop CS5 Configurator is useless and it won't work.

CS4 or CS6 beta works ok.

Paul - are those your scripts on that link you submitted?

It sound like your creating a collage type poster. For that you could create a template PSD a layer stack of various sized images. Its easy to create such a stack.  Just open a new document 2' x 3' at the DPI ypi want to print at. Add a new empty layer the use the rectangle marquee tool to make a selection the size you wand an image to be like 4"x 6" and fill the selection with a slod color like gray.  Repeat adding other layers make selection the other sizes you want an fill these.  When you have the number of images you want in your poster you can use the move too lwith auto select to position and layout images placements.

This layer stack can easily be converted into a Photo Collage Template and populated with your images using scripts in my free Photoshop Photo Collage Toolkit.

There are ten scripts in this package they provide the following functions:

1. TestCollageTemplate.jsx - Used to test a Photo Collage Template while you are making it with Photoshop.
2. CollageTemplateBuilder.jsx - Can build Templates compatible with this toolkit's scripts.
3. LayerToAlphaChan.jsx - Used to convert a Prototype Image Layer stack into a template document.
4. InteractivePopulateCollage.jsx - Used to interactively populate Any Photo Collage template. Offers most user control inserting pictures and text.
5. ReplaceCollageImage.jsx - use to replace a populated collage image Smart Object layer with an other image correctly resized and positioned.
6. ChangeTextSize.jsx - This script can be used to change Image stamps text size when the size used by the populating did not work well.
7. PopulateCollageTemplate.jsx - Used to Automatically populate a Photo Collage template and leave the populated copy open in Photoshop.
8. BatchOneImageCollage.jsx - Used to Automatically Batch Populate Collage templates that only have one image inserted. The Collage or Image may be stamped with text.
9. BatchMultiImageCollage.jsx - Used to Automatically Batch Populate Any Photo Collage template with images in a source image folder. Easier to use than the interactive script. Saved collages can be tweaked.
10. PasteImageRoll.jsx - Paste Images into a document to be print on roll paper.

Documentation and Examples

Download

There is no need to resize and crop images the scripts automate all of that by sizing images to fit the layout  slots and mask them to the layout slots aspect ratios.  Any image can be fitter to any layout slot. However for best results and composition images should have the same orintation of the layout image slot an images is being fitted to.  While Landscape and Protrait image can be fitted to a slot with the other orintation the result look like a centered crop the composition change will be drastic when a landscape it fitted to a portrait slot or a portraits is fitted to a landscape slot. For maximun customization there is an interactive script"InteractivePopulareCollage.jsx"  that lets the user selet the images for the slots and tweak the automated placed selected images and text.

A full batch automated process is going to be better then a manualy one.  Once a Collage is populated it is still a layered Photoshop PSD so you can still manually tweak the automated collage. I think the documentation clearly showed that when I populated this collage automaticly

the manualy changed it to these other compositions

I guess then you walk and don't drive for its simpler to walk it just takes longer. You will see fewer places in your lifetime doing each step manually enjoy smelling the roses....