What is the shape of an acacia?

In your family there are probably different members, and although they share some off the same family features they do not look exactly the same. Quite different normally.

The same applies for Acacia's. "Acacia" is only the family name. In this family there are different members all having their own physical apprearance.

A flat top Acacia, or an Umbrellathorn have flat tops and stand out and so are easily recognised.

Knobthorn Acacia and the Fever tree are both in the acacia family but look completely different. Just as an example.

Furthermore do you need to look at the habitat they are growing in. What soil tipe is found, what is the climate (determening how easily they will grow etc). Then you also have to take into account the foraging of animals.

In an area where there are many browsers the trees will look physically different from the same tree species in a different area.

One of the animals which has a big influence (without destroying the tree like elephants) on the shape of trees are giraffes.

So I hope that it is clear that we can not answer your question. Another question might be what are the characteristics of Acacia's (if you would like to be able to recognise them).