An indicative that is not dependent on the model's size is the cubic wing loading, which may be calculated by dividing the weight in oz by the wing area in sq.ft raised to power of 1.5

Different types of model aircraft may have different cubic wing loadings (oz/cu.ft) as shown below:

Sail and Park Flyer: 4 to 7
Sport and Trainer: 7 to 9
Pylon and Scale: up to 13
Electric Ducted Fan: up to 25

To calculate the cubic wing loading click [link=http://adamone.rchomepage.com/calc_cubicload.htm]here[/link]

The recommended max conventional wing loading is much dependent on airplane’s size.
For instance, a full scale Cessna 152 has a wing loading of about 167oz/sq.ft, a model airplane with such a wing loading could hardly be able to fly.
However, the full scale Cessna has a cubic loading of about 13 oz/cu.ft, which puts it at the high end of a scale model category regardless of size.