I don't have a Polo radiator (I have Mercedes 190E), so I can't give you the exact size, but as a guideline, I would recommend that the diameter of your fan should be as close as possible to the short size of the radiator's core.
For example, if your radiator's core is 300mm x 250mm, then the fan diameter should be as close to 250mm as possible (10'').
Puller fans are more efficient than blower fans, and it is recommended to install the fan behind the radiator rather than in the front when you have the room for that. We do have enough room on the Roadster with these small OEM radiators, the font frame being leaning backwards.
There has been a discussion here last year about ducted and non-ducted fans. I still think that a good fan do not need to be ducted, as long as you make sure that it sticks to the radiator's core (no gap between the fan housing and the core.) It also will not need as much room as a ducted fan.
Hope this will help
