In my experience with stairs the weld details are usually drawn by a draftsman that may have little or no knowledge of welding. An engineer may or may not approve the prints. This is not the it should be but that has been my experience. It depends on the fabrication company. Some do a poor job in this area. Some do okay. The joint configuration you described would be a flare bevel and hopefully not a fillet weld. The 3" long welds may be up the inspectors judgement. Post would have less wobble if welded on top and bottom. Inspector may not like it though. If you put 3" centered in the 6" lap the inspector would not reject unless detail contradicted the welds on post. I always erred on the side of too much when it came to stair safety. I have put many rails and stairs in army barracks, middle, high schools and colleges and can invision ten kids horseplaying pushing and pulling on the rails then I add more weld. As far as the size goes a fillet gage or a scale can check the size on the post side using an imaginary line.
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