Hello Tony, a ledged (not legged) and braced door is pretty much one of the easiest types of door to make, I've made many types of doors in my time.
As to the sizes and specs of the door you wish to make that will depend on where you are using it and wheather it has to match exsisting, finish and so on.
The one in the shots below is only 2 feet wide but construction is the same for a wider door.
Starting point is the opening that you have. The lenght of boards is goverened by the height of the frame. I always prefer to have it slightly longer to allow for the frame being out of level on the head and cut it to fit on site. Width then is goverened by the opening width. It's always a better idea to divide the distance up so that you are left with boards of the same width. As I have said in the picture the door is only 24" so each board is 6" wide. If it is inconvient to divide it up equally try to have the two outside boards ripped down to the same width.
The boards are usually tongue and groove with a vee mould where they meet. If the door is to be painted it's a good idea to paint the tongues before assembly, then if the boards shrink you are not left with a unsightly line.
To construct the door the boards are placed face down on a bench and the ledges put on the back of the door. These can be glued and screwed or nailed to the boards and the holes filled or alternativly lay the ledges down on the bench and fix the boards to them either face fixed or secret nailed through the tongues. I have made these types of doors for historic buildings and in some cases have used oak or elm boards and nailed through the face with larged headed cut nails and clenched them over where they have come through the backs of the ledges. It's all a case of the right door for the building. The ledges themselves do not go into the frame rebate so should be left short of the edge. Frames for these doors have shallower rebates than "usual" doors. In the picture the boards and rebate is about 7/8".
I always like to space the ledges as one would space hinges on a door i.e. the top ledge is closer to the top than the bottom ledge is to the bottom of the door. Usually hinges are 6" down from the top and 9" up from the bottom.
Once put together it's then an easy task to fit the braces, the lower end being on the hanging side.
For ironmongery I prefer simple tee hinges and catchs such as a suffolk latch.
Good luck with the door and I hope this hasn't been too long winded.