As the name implies, it has to be small.
I want all the components to be on the same side, so it's more convenient to solder by hand (as everything is SMD). It's also better if I want to use that module in future designs.
For all the "Low voltage" components, I used 0603 components. They can be tricky to solder by hand, but it's doable. For the high voltage 10nF onboard cap, I used 1206 size (because I had no choice). I may have to use an externah HV electrolythic capacitor, as 10nF may not be enough.
For the transistors I used SOT-23 types. The PNP transistor is a S8550 general purpose transistor, and the High voltage switching transistor is an MMBTA42 300V transistor (the best I found with a great voltage/current ratio), and it's also why the µHVPSU v1 is limited to 300V.
The rectifier diode of the converter is a HER108, because, reasons.. (Okay, it's the one used in the original design).
Here are some pictures :
(3D renderings made with this tool (don't pay attention to the green color, the real ones will be white))