The board looks like it's designed for easy hand-soldering and modifications, which might be the point, but I wouldn't call it dense at all, except maybe by 1980's standards
. I would start calling it dense after squeezing it in about one third of the current area.
Personally, I mostly design in this same density level unless I absolutely need to go denser, for the sake of prototyping; I tend to need to change some resistor values, cut traces, add fix wires etc. When I need to, I use 0402, since it's still easy to hand-solder; but being slightly harder than 0603, there is no point in using 0402 in the majority of the cases when it's not needed for any reason. But for example, on 0.5mm QFP CPUs with multiple GND and Vdd pins, using 0402 bypass caps instead of 0603 is a HUGE help. So my boards tend to have the passive case distribution of about 10% 0805, 80% of 0603, and 10% 0402. I even may have mixture of 100n (edit: 0.1u for those from the USA
) ceramics in 0402 and 0603 sizes, which would be stupid for machine assembly, but for a human, it's nicer to have 0603's when there is space.
I agree that you don't need 4 layer board for this kind of simple project, where cost is also important.