Unfortunately, there is no way to adjust the Diamond icons' size due to the limitation of the old GUI library. However, user has the option to adjust the font size/style of the Source Editor (Tools > Options... > Source Editor > Font). User can also Zoom In on the Project Navigator display (View > Zoom In) but note that this will not adjust icon sizes. Furthermore, user can also try adjusting their machine's display settings and scale it up to 150% or more. It should render Diamond a bigger display.