Currently I have two standardized containers in use, the TinCan™ of 19x9x9 with 15x5x5 internal volume (375m³) and a tare mass of ~50 tons, and the DoubleChest™ of 53x27x27 with a nominal internal volume of 20k m³ (actual volume is 47x21x21 , 20727m³) at ~1000 tons empty. Note that those are actual cubic meters, not cargo units, as we use custom config values, so multiply that times 100 for default.
Both variants have active (rail docker) attachment points on the top and bottom; the DoubleChest uses USD ports exclusively, the TinCan comes fitted with USD ports at the front and back (and could replace the single-points top and bottom with USDs if required). Logic controls are in place and can be configured to pull/not pull when attached.
Either model can stack two short to a long side, and eight TinCans can fit inside the volume of a DoubleChest, or eighteen within the same dimensions (not that it would make sense storage-wise, but it could be done). Both containers have rudimentary manoeuvring thrusters.
(note that in the first image, those are two prototype versions of the TinCan)
Barrel and pressure tank versions are in the queue as well.
So naturally those are my preferred dimensions ;)