you normally control the (relative) sizes through the layout, e.g.
ui := DynamicComposableModel new.
ui instantiateModels: #(text TextInputFieldModel btn1 ButtonModel btn2 ButtonModel).
ui btn1 label: 'Button 1'.
ui btn2 label: 'Button 2'.
layout := SpecColumnLayout composed
newRow: [ :row |
row
add: #text;
add: #btn1 width: 60;
add: #btn2 width: 60
] height: ComposableModel toolbarHeight;
newRow: [ :row | ].
ui openWithSpecLayout: layout.
A lot of effort has been put into it recently to update it, so it should be up-to-date.