Binding Combo Boxes

I have a windows forms program linked to an Access dB for an Inventory Management system.

The dB has Tables for

• Suppliers - SupId, SupName, ...
• Inventory - InvId, SupId, InvCode, InvDesc, InvSize,...
• Size - SizeId, SizeDesc
• Colour - ColourId, ColourDesc
• ... 

I have created a DataSource in the program from the dB and (simple) Relations between the Tables

I have used the Toolbox tools - DataSet, BindingSource to create the initial databindings between Supplier and Inventory but am struggling to get the link between Inventory and Size working.

My Form has;

1. cboSuppliers for Suppliers (no Problem show Supplier Name)
2. cboInventory for Inventory (based on SupId) that loads the Inventory Codes for the Selected SupId (no Problem)
3. cboSize for the Size of the Inventory Item (Problem)
4. other text boxes/checkboxes that show the other details on the Inventory item

How do I provide the databinding to make cboSize show the associated SizeDesc (from Sizes Table) for the selectedItem from cboInventory?

Thanks in advance,

Rinso