I suspect that your model looks something like this:
Perspective (or Window)
PartSashContainer
Part Stack
Part (Welcome)
Part Stack
Part (Table)
You can use the Attribute "Container Data" on each of the PartStack Elements in your Model.
For example you can have the Table-Part use 70% of the available space:
Perspective (or Window)
PartSashContainer
Part Stack [container data=30]
Part [id=welcome]
Part Stack [container data=70]
Part [id=table]