Reynald's number
Imperical results are that wing loading is quite simply of more importance in these smaller sizes than any change in airfoils .
For example - the case mentioned is 12/15 (4/5) the wing chord -or about a 20% shift in chord.
Having built a number of a design - then increasing or decreasing them - we got a lot of hands on exposure to what happens .
Typically - in this size range - --if the model had a 30 oz ft loading at 15" chord-- the same power off characteristics for the 12" chord - required that the wing loading drop below 25 oz ft-
Power off was used as the comparison basis as power on comparison is simply not predictable
stalls and landing approachs show when the model starts to loose control- snaps show how easily the model regains control - this testactually shows the most striking differences in wing loading .
comparisons
Dropping sizs even further to small models with chords of avg 9" show that wing loading now must be in the mid teens - to "equal" the character seen at the larger sizes.
If these numbers sound strange to you - try some comparisons -if possible ,with models .
I really don't know what the shape of the curve would be on a graph- in this size - and really there is probably not a lot of referrence stuff.
For working this up- -Imagine all speeds stay at 60 mph and chord changes from 15-12 -9" avg on a wing with a 5-1 aspect ratio- your choice of airfoils