I don't know what size it is supposed to be, but the Toshiba Tablet website: Toshiba Tablet (under the "Specs" section, Ports & Expansion) states the tablet has: "USB, mini-USB, HDMI, SD slot, and a headphone jack. I doubt if it would be other than the standard size mini-jack...
The very website you link to (in the 360?) is where it states that the headphone jack is 1/4" and not a 5mm jack. I don't even know where I would go to get a 1/4" jack. I hope that there is a 5mm option of a converter that comes with this tablet.