The final size of your graph depends amongst other things on the size of the type and the number of digits in the XY values. So rescaling with either the Scale ot Transform tools is almost always necessary. I suggest snapping ruler guides to a rectangle which is in the correct size may be a good idea. Then you can always snap your graph to fit to the guides. Be aware that simple scaling will result in the type becoming distorted. You can select the type by double (or treble)-clicking on one of the digits with the Direct Selection (white arrow) tool. That way you select the whole group to adjust the size of the type.
Select the graph click on the graph tool in the tool bar there is an x axis settings for the drop down gives you some control.
So does the columns width settings.
You can also follow this:
Select the graph with the selection tool (v) -> Right click -> Scale-> uniform selection, input as 200% to double its size or 120% to increase the size by 20%. Click OK.
This worked perfectly. Better than all the other options.
Thank you so much ankitas!
I found that ungrouping the graph and then resizing as you would with a normal shape also works, but this makes it so you're no longer able to edit the data.