What are the measurements for a "Black Box"?
The two main "black boxes" are the Flight Data Recorder (FDR) and the Cockpit Voice Recorder (CVR). These units are available from various manufacturers and therefore sizes vary. Many units can be used as either CVR or FDR, or as both.

A quick search provides a few examples:

etep Sentinel ED-112: CVR/FDR
Height: 3.929 in (9.980 cm)
Width: 4.970 in (12.624 cm)
Depth: 6.907 in (17.545 cm)

L3 F1000: FDR
Height: 7.62 in (19.4 cm)
Width: 4.88 in (12.4 cm)
Depth: 19.56 in (49.7 cm)

Universal Avionics Systems CVFDR: CVR/FDR
Height 6.1 in (15.5 cm)
Width 4.9 in (12.4 cm)
Depth 8.0 in (20.3 cm)

L3 SRVIVR: CVR/FDR
standard configuration:
Height: 3.25 in (8.26 cm)
Width 5.55 in (14.1 cm)
Depth: 6.55 in (16.6 cm)
extended configuration:
Height: 3.25 in (8.26 cm)
Width: 5.55 in (14.1 cm)
Depth: 7.2 in (18.3 cm)

L3 FA2100: CVR
Height 5.5 in. (14.00 cm)
Width 5.0 in. (12.70 cm)
Depth 12.6 in. (32.00 cm)

If you were asking something else, see:

How much data do FDR and CVR generate on commercial jets?

Or other questions already asked here.