there is one factor you can use. this factor is 1.36 which is the difference between projected and surface area.
e.g. Wipika Airblast 10.0 has 13.6 surface area.
Following companies use projected area:
Wipika, Cabrinha and Takoon.
all their sizes times 1.36 and you have the comparable size of other producers.
for the most companies you can see their sizes and the surface area at http://www.kiteproshop.com
This means if you are looking at the Airbalst 11.8 it has 16 sqm surface area.
this is quiet big to start with.
If you want two kites, a very good combination would be:
9-10 and 13-15 sqm surface area.
The Naish ARX 9.5 and 13.5 would work well together.
Or you go for three kites:
7.5 - 11.5 - 15.5
that's what I use. But also depending on your weight, board and winds.