There are 3 rules of thumb that will get you started and keep you out of trouble. Of course there are situations where these 'rules' are meant to be varied... but they will cover 95% of your M/T needs.
1. The tenon thickness should be about 1/3 the thickness of the stock being mortised.
This is to ensure sufficient side wall strength in the part being mortised.
Note: This is not 1/3 the thickness of the part with the tenon... that will be the case only when the tenon stock and mortise stock are the same thickness.
So... a 3/4" piece being mortised should have a 1/4" tenon fit to it.
2. The tenon width should be no more than 5 times it's thickness, with a maximum width of about 4".
So... that 1/4" tenon from #1 should be no more than 1 -1/4" wide.
3. In a blind mortise, as you were referring to, the depth of the mortise should be about 2/3 of the width of the part being mortised. The deeper (hence longer the tenon) the better for strength... but a practical limit is reached at about 2/3 the width.
So... a mortise into a 3/4" wide part should be about 1/2" deep.
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