although I have found info on the mattel oem part (H4446) I did not see specific size data.
anyway it's about ~3.5-3.7mm center positive. a 5V part will work but you'd do better with a higher quality p/s in that case e.g. switched.
psp and some pocketpc adaptors will work. powering from usb seems to work. see also. many home routers have a 5V center positive jack, altho most of them seem to be too large (i.e. >4.0mm)
the plug from my iriver (3.4mm od 1.3mm id) is too small in terms of the inner diameter.