How to edit symbol diameter related to the catchment area?

I have a table with column Catchment area (in km2), and I need to set symbol diameter based on that column. I need to get .sld file so I'm working with uDig but if you know better software for this, I will accept your advice.

EDIT: I found out how to do it, but result is not acceptable because symbols are too big. There is no any normalization option in uDig or QGIS.

Thank you.

When scaling point symbols, it's good to scale them in a way that double value results in doubled symbol area (not diameter). Therefore, one has to use the square root of the value as a multiplicand.

While calculating square root, normalization can be applied at the same time e.g. using Field Calculator in QGIS.

Update:

The Field Calculator query would look like this. (Replace x with the biggest area value in your dataset.)

sqrt( area_field ) / sqrt( x )

You can then create Attribute-based point styles.