5cm (W) x 3.3cm (H) x 0.8cm (D) -- width may vary with some cassettes (e.g. Buzzsaw is 5.1cm wide and Ravage is 4.8cm).

This might also help, dimensions for the Transformer cassette case:
5.7cm (W) x 4cm (H) x 1.2cm (D) -- the pegs for the spools are 1.25cm from either side of the case.