Hi Pav,
There really aren't any rules at all, luckily. If I had now similar space, budget and conditions for example, I'd go for 100x50x40(h), as I like tanks to be more on the shallow side but also with a large footprint to play around with depths and hardscape.
To follow up from above, above 45cm of height you'll also start to need high intensity and penetration lighting systems.