If you mean in the spec description class side, then it is :
newRow: aBlock height: aNumber
newColumn : aBlock width: aNumber
If you mean when the window is opened, how to resize the subpanels, you need to add splitters between the panels. Ex :
^ SpecLayout composed
add: #firstItem origin: 0@0 corner: 0.5@1;
addSplitter;
add: #description origin: 0.5@0 corner: 1@1;
yourself