Wikipedia says
The Mini-ITX standard does not define a standard for the power supply ...
Which means size and layout of PSU depends only on the case.
... though it makes some suggestions of possible options. Conventionally Mini-ITX boards use a 20- or 24-pin "original ATX" power connector. This is usually connected to a DC-DC converter board which in turn is connected to an external power adapter. Generally both the power adapter and the DC-DC board are supplied with the case.
An Intel document suggests
Provided the very different needs of the platforms that fit in the small desktop form category, this specification does not attempt to define a standard for the power supply to match with the Mini-ITX motherboard specification. Instead it recommends that manufacturers consider using a:
- SFX, TFX or FlexATX are available power supply form factor for chassis greater than 5 liters
- External power adapter for chassis less than 5 liters