I never thought to measure one. Wikipedia gives recommended range of dimensions for 400 oz (12.44 kg) London Good Delivery bars.
Length (top): 210–290 mm (~8.3-11.4 inches)
Width (top): 55–85 mm (~2.2-3.3 inches)
Height: 25–45 mm (~1-1.8 inches)
https://en.wikipedia.org/wiki/Good_Delivery
Here's another reference that narrows down the dimensions on page 2. On page 3, however, it lists the dimensions of LGD bars from various refiners all over the world and there's quite a bit of dimensional variation.
http://www.goldbarsworldwide.com/PDF/BI_6_LondonGoldBars.pdf