The poster claims it isn't as long as Nintendo Wii's Nunchuk. The wand has “one very big button on top" with all four buttons and a 'T' trigger.
This is all rumour of course, though VG247 points out that sources of theirs claim more or less the same thing has NeoGAF poster 'ichinisan'.
"Nunchuck has an analog stick at the front, below that X and O and below that a D pad. Underneath is L1 and L2. Its actually long, not like the Wii Nunchuk," they posted over on the NeoGAF forums. "As for the Wand it has one very big button on top and then all four facebuttons (X,O,Square and Triangle) and underneath is a big trigger called T."
Sony will be showcasing PlayStation Arc at this week's GDC and they have an off-site press conference this Wednesday, so there's plenty of time for us to find out.