If you are using an 'Image' field, and want to control the size of the image that base_edit or base_view will show, then you can do the following:
1. Define a 'preview' scale in your 'sizes' property on the widget ('sizes' tagged value in ArgoUML). The image widget uses this scale (if available) by default.
2. Set the 'preview_scale' property on the widget ('preview_scale' tagged value in ArgoUML) to an existing scale/size name. The image widget will then use this scale instead.