Great effort Brian. As a "competitor" with the smallest Arduino Duemilanove compatible device (om328p), I understand what it takes to bring something like this together. The "pico" is only a fraction of an inch smaller than the Nano. The om328p is 0.2" shorter at 1.4" by 0.7"
Personally I prefer the "stamp" style layout for TX/RX/Vin/Gnd/Reset/5V but understand you made some compromises in this area. Also it would be good if the A6/A7 pins were at minimum aligned with the other pins and best if they could also be used with a breadboard. It would take some shifting around but I know it all fits having a similar board layout using the SOIC version of the FTDI chip that is 1.5" long.
The pico is missing a regulator but does bring out the AREF input. I understand that most people will power it from USB but a regulator is nice for stand-alone use.
Your website mentions that the Pico can be used from 2.7V to 5V. This is not correct when using a 16MHz crystal. The ATmega328p is only rated down to 3.6V @ 16MHz. Also the FTDI chip needs a minimum of 4V to operate.