Can you get dimensions to show on light table?

I am having putting photos on light table to fit a collage frame. Does anyone have any suggestions?

I do not know of any option to show the dimensions on the light table in addition to the grid. But you could take a photo of the collage frame and put it onto the light table as a backdrop. Then arrange your selected photos on top of it.

And then print on a paper with the exact dimensions of the backdrop.

Light Table is probably not the right tool for any job that involves dimensioning. It great for sorting and viewing and conceptualizing, but is not useful for dimensioned layout (IME). I recommend using it for dimensioned layout _only_ if you can use the grid, and that use provides the precision you need.

Remember that your digital images have no real-world dimensions before you make a real-world object (e.g.: a print) or assign a real-world unit to the digital units used for digital images (e.g.: 124 pixels = 1 cm).

Can you specify what you want to know at the end of the exercise? Do you just want to know which picture goes in each mat cut-out? Do you want to know where to crop Images in order to fit the mat cut-outs? Are you changing the scale of the Images to fit the mat?

How are you going to convert your data to stuff (i.e.: print)?

The problem can be complex. If you are comfortable with Photoshop, do it there, assigning real-world dimensions to objects (e.g: make a rectangle 24" x 36"). Put the mat on one layer, and each image on it's own layer. Resize and position as desired. If possible, print the entire sheet (excluding, of course, the mat) from PS (carrying the scaling information back to Aperture is toilsome). (This is, incidentally, a good way to arrange pictures on walls {that is, already printed (and matted and framed) pictures}. I use it for gallery layouts.)