How high should centrepieces be?

I think it's personal taste really and depends on the venue..

My venue ceilings are quite high in the marquee so fishbowls would be completely drowned out but in the evening room the ceiling is low so i've gone for smaller ones..

I think it is up to you. Just make sure that if they are low that they really are low so that people can see the other side of the table over them and talk to everyone or if they are high that they are high enough not to block anyone's view. I hate sitting at a table and having to peer round to talk to people.

The main rule is to remember that they are either above or below eye level. Low table arrangements can make just as big an impact as high ones.

I had quite high center pieces, they were 1.2meter oberlisks with ivy twisted up them, led lights and white coranations.
They looked so effective, everyone commented on how different and pretty they looked. They said the room looked like something out of a fairy tail so i was really pleased.

I had never seen them before either, i just had the idea when i seen the obelisks in my local garden center.

The picture isnt as effective as it doesnt catch the lights but they did look really beautiful. We were in a big room though with 200 people and VERY high ceilings. Its personal choice and budget though