I think I understand what the OP is asking, and I've toyed with the issue myself. I think the part of the bed that slides out is going to have to be shorter than the stationary part. The amount will depend upon what you make it out of. For strength, the easiest will be to make it out of 2x4 material, but if you were willing to spend the money, you could easily make it out of 1x 4 oak. to get the dimensions, get a few scraps and play with them. First figure out what your overall dimension needs to be, then start with a piece for the fixed side a little longer than half, and a piece for the other side about half the width. Remember that they will need to overlap, and you will need a connector for the sliding section that sits under the fixed section, to hold it all together. I'm not sure if my description is making sense, but if you Google images of sliding slat beds you should be able to decipher the details. The dimensions you'll just have to play with until you get it right for the size bed and typ of material you're using, as it will vary for every difference.