Can more or less make it any dimensions you wish. The outside dimensions of my closed box are approximately 12-1/4 inches long, by 9-1/2 inches wide, by 5-3/4 inches deep/high. The inkwell is actually a holder for a 3-oz bottle of Noodler's ink. In addition to the link at azWoodman I posted above, do a search on the 'net for "writing slope" and you'll end up seeing a bunch of old antiques that are just beautiful, mostly in the UK and Europe, and with very high prices. Ebay will show up with a few, mostly in the UK and Europe, and limited shipping to the US - and shipping costs could be as much as or more than the slope itself! Further, you won't find many really nice, ready-to-go slopes on Ebay. Most will require a lot of restoration and/or refinishing.
There are a couple of keys that make a box writing slope not so simple. First, the lid is angle cut so that the back down side is even in the middle of the box, and the front up side of the lid is about 1 inch total. This way, it can open to an even-sloped writing surface. The panels inside that pull up are not attached to the box with hardware, but actually uses the applied writing surface (leather, felt, velvet, etc.) as their hinges. They fold up to the middle. There is an additional fold in the very center between the panels where the box folds closed. There is a slight gap in the back when closed, allowance for the folded writing surface inside. The hinges for the box itself to open need to be side hinges, not hinges on the center back of the box. When opened, the back edges of the box come together and your writing surface will lay over them. The edges of the panels may need to be beveled slightly, in order to lift up and down without friction against the outside edges. You'll have to add rails inside or some other support for the panels inside the compartments. And turn buttons to keep the panels in place when you close the box.
There is a how-to book available on Amazon that has a writing slope in it. If I can find the right one, I'll post a link to it. It is, I think, a bit more rustic than my modern slope (which I finished to look old and well used, heh). Edit again: here is the link to that how-to book, and the slope is called a Cottage Writing Box. The book is called
Boxes and Chests. And here is another link to the same book that actually shows some of the plans of the box itself:
Cottage Writing Box
. They do leave out some details, and it is copyrighted material. I think its a safe bet that buying the book, as inexpensive as it is, would be a good investment even for a rustic box.
Now, as for just a simple, angled writing surface... two pieces of sturdy hardwood attached to each other to create that angle according to the height you prefer, and then covered with the writing surface of your choice, would probably be just fine.
When I ordered my writing slope, it was all learn-as-you-go, as all we had to go on were pictures of antiques I'd found on the 'net. It was a lot of trial and error, but it paved the way for more beautiful slopes to come along. Mine is very simple, yet it was quite a project originally. The dimensions finally decided upon allowed for 8-1/2 x 11 inch paper to fit in the bottom compartment of the slope. There was a slight modification needed, though, so I don't store 8-1/2 x 11 paper in the box, but I do have writing pads and envelopes stored.