Adjust number of classes when symbolising by both type and size

[HelenCooper]

Hi, 

In MapViewer, I want to symbolise a layer using two fields, one is a category type, the other is an amount. 

(as per example below):

I don't seem to have any control over the number of classes and break values (unlike if I were just symbolising by an amount field alone).   Am I missing a trick somewhere?

Many thanks, 

Helen