I'm trying to build an art exhibition that fits into a 5x5 meter square. I have rectangular boxes that I want to set up in a pattern. So I want to make a grid that's made out of rectangles on my design. Is there a good way to do that in Photoshop?
I'm a little confused as to what you are asking. I don't know if you are interested in (1) having one picture that is broken up into cells in a grid with a gutter between cells, (kind of like when you see one image across multiple screens at once) or (2) if you are looking to fill the cells with different images.
In any case, you can use the regular rectangle frame tool and click and drag, but while you're dragging, use the left & right arrow keys to add more cols or up & down arrows to add more rows to your grid. When you let go the mouse, you'll have a bunch of frames (cells) displayed as a grid.
At this point, if you are interested in option (2), you can open the folder with your images and drag them into the separate cells individually.
If you are interested in option (1), you can then (while having the grid of frames all selected) use Ctrl+8 (Windows) to Make Compound Path. You can still move the individual cells around or even resize or delete some to make space for text or whatever.
Then, just load the image you want to use. Copy the image (Ctrl+C) after it loads, then delete it (Ctrl+X). And, select the frame grid and use the paste into command (Ctrl+Alt+V). You'll end up with one image distributed across all the cells.