Q: Is a standard first player Famicom controller the same size as a standard NES controller?

A: No, not quite. The shell of an NES controller is slightly thicker, but the pcb is identical.

As long as you follow the 4021 pinouts you can do whatever you like. Nice to hear you did the Hudson Joycard mod! That's a wonderful controller. Just word of warning though: the Hudson Joycard is MUCH bigger than a Standard Famicom controller. The Joycard was build for adult hands, while the Famicom pad was build for the hands of children. Since you already have a Joycard, there's really no point in doing a standard Famicom controller to NES mod, but whatever suits your fancy.