Michael -
I haven't done what you're trying to do but did some digging on your behalf. As I thought, what you're trying to accomplish is not playing to Photoshop's strengths but rather to a page layout program like InDesign. You can batch process the images in PS to a particular size but dropping them into a PS document in a grid layout is a fairly laborious process - especially if you're dealing with small images in a large format poster. You would really have to create the grid (view->new guide layout->dialog box to spec), then drop the images one-by-one into the document, resize (if not batch processed as above), and clip to each individual layer you create. Each layer would have a single image.
Here's a video on an InDesign image grid layout that I think what you're aiming for - bit more automated:
Depending on the number of images you have, maybe there's a service that can inexpensively do the layout for you. InDesign, unfortunately, isn't in my bag of tools right now.
As a side note, the term "large poster" is fairly generic. As someone who was in print production, I've worked on posters that range from 19 X 25 inches up to convention panels that run from 220 inches across to 96 inches high. Done in sections but on large format printers or even sheet fed machines up to 60" in width.
Also when spec'ing sizes, make sure you note if it's in metric or imperial - inches or centimeters. I've fallen into that trap myself when working with vendors across The Pond. Embarrassing to say the least.
I'm not a power PS user so some others may drop in with a greater perspective. Let me know if there's any other way I can help.
- Jeff