Research suggests it was available from LSL (a four line script is the rumour), but unfortunately llSetPrimitiveParams([PRIM_SIZE, ]); creates a 10x10x10.

Patching the XML allows you to size them out to an size you like, people also see the huge prims, but as soon as you close the edit dialog or attempt to change from cube to another shape they collapse.

Right now I am looking at various overflows on llSetPrimativeParams, it may be there based on the rumour of it being script based.