Image via gasolinealleyantiques.
Each plastic figure measures around 1.25" tall and contains a metal ball bearing on the bottom which enables them to roll. They are hand-painted, although not always with much artistic flare.
Images via toyhelper.
A "Stunt Speedway" track was sold separately in which the figures could do "loop the loop" action.
More Marx packaging.
Image via Hake's.