Hi,
the FCC in America will not apparently give permission for satellites smaller than a cubsat (10cm cube) due to the difficulty of tracking them, despite the obvious evidence that they can track something 1/8th that size just fine.
Yes, Radar uses "millimeter wavelengths" so at least "military grade" equipment should be able to track anything significantly larger than that. The only issue with a (metal) cube is that its flat sides may reflect the radio waves away in other directions, rather than returning them directly to the (Radar) transmitter. But small satellites are not going to have a stabilised orientation, so will tumble and at least sometimes be "visible" (to Radar, if not optically).
However, to predict the time (let alone location) of "re-entry" is very difficult because of the unpredictable drag of the upper atmosphere. The
Humanity Star satellite re-entered about
5 times sooner than intended/expected, because of its elliptical orbit (i.e. briefly dipping into the upper atmosphere once each orbit) and its unusually large diameter to weight (i.e. drag/mass) ratio.
Cheers, Alan.