Yes good point.
If your referring to the schematic posted, that's why there are five 1206 resistors to make a 100 ohm (100ma), and a 1206 for 1k (10ma), the 10K (1ma), 100K (0.1ma) and 1Meg (10ua) are 0805 types.
A lower reference voltage becomes a tradeoff with the errors due to IR drop in the board, various jumper & contacts and also the offset error of the op-amp. With a 10 volt reference these effects are reduced and why the decision to go with 10 volts from either ADR01 or scaled LM399.
Also tried to stay with parts that I had or weren't expensive, since this isn't intended for serious use. The PCB hasn't been fabricated, since another version is in the works that includes an oscilloscope calibrator but does away with the high-precision voltage divider in the upper right and removes the LTC1043 to keep the PCB size down.
Best,