Not enough info for an answer to your question. What are you building? How big are the components you are joining? What is your "high load weight"?
I don't know if there is a formula for the size of mortises and tenons, there probably is since there seems to be a formula for just about everything. In general, I think it depends on the size and requirements of your project. A 1" X 3" X 1/4" mortise and tenon might be fine for joining a 1 X 6 to a 2 X 4 to support a 200 lb. load, but not so great for joining a 4" X 12" rail to a 6 X 6 style to support a ton. You can probably rely on your gut. If it looks strong enough, it probably is.
My $.02.
Hank
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I generally make the mortise depth around 1/3rd of the width of the piece being mortised, you could go deeper, actually through it; and the thickness of the tenon 1/3rd the thickness of the piece being tenoned. I don't have any source for this except for a crusty old guy named Gus (he's since passed) who told me that 30 years ago. Works for me, have never had a joint fail.
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Probably are a bunch of rules - by any number of fools. In your case, the load will be borne on the endgrain of the legs - almost always the truth - so the tenons are there to prevent rail racking. It's the shoulder that counts most in those constructions, with the tenon long enough to contain a pin to draw bore it and keep the shoulder in firm contact. In hard wood, probably ~3/4" with the draw midway or favoring the entry.
Fox wedged tenon could be shorter.
Better to follow the leader than the pack. Less to step in.