Can't make circular array around an empty I'm trying to make a circular array of objects around an empty, as most tutorials suggest however whenever I do this I get a load of weird random sized objects appearing.

It looks like the dimensions of the initial object changes to random large numbers and when I try to change them manually they just increase to even bigger numbers.

I thought this might be a bug in v2.78 so I uninstalled and got v2.79b but it still does the same thing. Here is a before and after selecting the empty as the object offset.
I may have missed the point of your question, or not understood what exactly you wanted to do, but ...

Ignore the empty and don't activate offset. Put the 3D cursor in the centre of the world (0,0,0)

Create a Bezier Circle

In the cylinder's modifier, change Fixed Count to Fit Curve.  Choose the Bezier Circle as the curve.

You'll get a vertical array of however many cylinders you need to fit the circumference of that circle.

With the Array selected, assign a new modifier - Curve - and choose the Bezier Circle as the curve.  I had to set the axis to Z to get what you see in the picture below.