Welcome John R.,
Here in the US the lumber sizes are standardized, but just not the standard dimension that is marked. A 1 x 4 is really 3/4" x 3-1/2". But even that varies 1/8" due to the high speed way that construction lumber is milled.
I'm using premium cabinet grade clear red oak that is square edged (not rounded) and is dimensionally very accurate. We can buy similar boards of poplar that is lower cost than the red oak. Oak is stiffer and looks better.
David's plans have no errors that I have found. Some of the upgrades like 1/2" lead screws will require you to make the shaft holes larger or possibly shift the position of the shaft hole slightly if you use anti-backlash nuts.
One easy no cost upgrade is to make the end plate that the x axis stepper mounts on double thickness (same as the side rails) when you are making parts for the frame. Use correspondingly longer bolts for the stepper and idler. A 1x4 board flexes too much.
Check the other Solsylva 25x37 build logs here for some ideas. If you have questions, just ask.