Here are the different things I can use for a lap desk:
- My normal plexiglass sign which lets me both slide layouts inside as well as a thin sheet of metal (which is kept in place by taping it to the piece of paper for the layout).
9x11.2 inches (101 square inches) - 9.6 oz without the metal sheet in it. 10.2 oz with it and the two sheets of paper in it. This works well but I can see an advantage if I could get good stability with something a bit smaller.
The problem is, how much smaller. Without having a variety of things to actually try, it's a lot of guess work. Which brings me to the other items I have so far. Oh, and none of these have the advantage of a layout view or metal to hold in place even if you set the desk and TB down on its side:
An old fiberboard item that was a shelf in an old computer desk. Not designed for heavy weight! But fine for the TB. This one is actually bigger at 11x14 inches (154 square inches), but even so is only a bit over 5 oz. If I were to use this, I'd definitely cut it down to no bigger than my normal lapdesk. People should keep in mind that what you type on makes a difference in feel. I've heard that having the TB on granite is really extra nice though you wouldn't think it would matter much compared to solid wood. But I'd say the fiber board has more flex to it than anything else I've tried and so would suffer some from that. And I guess you could more easily accidentally bend it. If I cut it down to the same size, the weight should come in around 3.5 oz.
I also have a 6x9 (54 square inches) - 2.8 oz with the clip removed plastic clipboard which is a little small for good stability. I'd guess that 10 inches may be okay but the real problem is with the width. As you type, it is going to shift to much, top to bottom for primary use. If I had it the same size as my main unit, the weight would be under 5.6 oz so pretty light.
I found the plywood version. It is 5 1/8 x 10 1/8 (51.9 square inches) - 3.3 oz. The length may be okay, but the width isn't good enough - worse than the clipboard since it is about a inch less. Making it the same size as my plexiglass unit would give it a weight just under 6.6 oz so still pretty darn light. And someone should be able to route it or something to put in a sheet of metal if you wanted that too.
I suspect the length needed may vary depending on the person's size and how they sit. Obviously, if you sit with your legs apart more, you need the desk to be longer. In my case, if my knees were together, the clipboard length may be okay, but since I don't sit that way, there would need to be at least another inch in length.
The width is going to be affect by how you type. If you keep the palm of your hands in the air, then you can probably get by with something narrower. But I rest the heel of my palm on the desk which pushes down on the bottom as I'm hitting keys higher up. This can cause a fair amount of shifting around (rocking). I guess you have to figure it isn't going to be rock solid like a desk, but you want some stability.
That is the hardest thing to judge without having more sizes to actually try. I know a width of 9 inches works. I know a width of 6 inches isn't good enough for regular use. The question is, were is the place size between the two. I'd guess at least another inch.
If that is correct, then, overall, I'd want something 7x10 inches if I wanted to minimize size but not suffer much on stability. To minimize weight, you probably only need to stay away from plexiglass SIGNS, since they essentially have a double cover for the paper to go between. That should keep you below 5 oz and maybe below 4.