Ainsty has a boxed set of three ISO style containers, I have been buying a box at shows over the recent years to build up a number of them.
They measure up approximately at 42mm high, by 39mm wide by 98mm long. This makes them approximately 1/62 scale (traditionally 28mm scale is 1/56 - it is however about 25mm scale).
A full size 20 foot container has the dimensions of 2591mm high, by 2438mm wide by 6058mm long (scaling to 46mm by 43mm by 108mm at 1/56).
Here is one with William Killian for scale.
The decals are MiniScene Models Graffiti Transfers (MG2). They have a white base so need to be trimmed closely. I followed the instructions, except I pre-dampened the application site with Humbrol DecalFix and applied it over the decal when in place, pushing it into the ribs in the container sides.
Once dried, it was varnished with Humbrol acrylic matt varnish. Then the there sides were treated.
Finally a last wash of Agrax Earthshade was applied.