Sure, happy to Sir.
These are done with a 1,1/4 inch bit. Probably about the biggest easily findable size for a brace.
The traditional option is to pre-make legs and season them so they get clamped in when the seat drys.
For spontaneous conctructions I drill nice and deep (about 3 inches if the material will allow it).
If you can taper the tennon slightly but leave a little space in the end of that mortice when it's tapped in, then as things dry the leg can slip deeper into it's home. Does that makes sense?
-If the seat needs to be much less chunky then it's a good time to think about drilling right through the piece and securing the tennon with a hardwood wedge. -A nice progession if you've not yet tried it.