[pjp]

Size isn't considered part of an object's style, but an object property, like it's placement.

However, there are a few ways to do make several objects the same size. The first is to make a multiple selection, making sure the first object your select is the one that's sized to your liking. Then use Arrange → Size → Make Same Size. This is also available in the contextual menu.

Alternatively, you can use the height and width fields in the Geometry inspector to edit the size for a multiple selection. These fields are also available in the ruler if it's visible (Command-R to toggle this.)

Hope this helps!