How to determine which size of surface mount resistor for an ADMP441
You can download the *.brd file and open it in Eagle, then look at the footprint properties for the footprint specifier.

In this board, it appears that the two SMT passives are 0603.

To generalize this answer, if you are not able to download the board files (say, all you have is Gerbers), then you can open the Gerbers in a Gerber viewer, measure the distance between pads, and determine which component size the board uses. However, it is generally a bad idea to try to populate and use a board for which you do not have a schematic or board files for.