This is the most relevant entry I found related to silkscreen rectangles in Flux:
A silk text is a particular kind of silkscreen that lets you add text to your PCB.
A silk line is a line that stretches between two specified points. You can use a line path rule for any desired curves.
A silk circle is a circle with a specified origin and diameter.
A silk rectangle stretches between two specified points.
The stroke length, spacing, style, width, and width of these three silk elements can all be modified using the appropriate rules."
Source
I could not find documentation describing a way to set an exact 33 mm by 20 mm size for a silkscreen rectangle. This likely means the feature is not available in Flux documentation. You can browse or ask here: