The skeg is basically a wing which produces a force due to the flow past it and the angle of attack of the wing. The angle of attack comes from the turning of the boat. The further the skeg is from the center point of the turn, the greater the angle of attack. The force creates a restoring moment to the boat (assuming the skeg is aft of the turning center) the moment is also greater as the skeg moves further away from the turning center. So to provide the same restoring moment, the skeg would need to be larger if mounted near the center. The amount of restoring moment required to make the boat feel stable is the big unknown; and likely varies from paddler to paddler and with the sea/wind condition. I generally put skegs about 1/3 of the boat length from the stern and find this to provide excellent results.

Leaning the boat does have a great effect on the effectiveness of the skeg, so your comment about being able to turn even with it deployed seems reasonable. I also usually install the skeg off the boat centerline for arrangement reasons and notice differences between turns towards or away from the skeg side, particularly when leaning the boat. I suspect that the skeg gets "shadowed" by the boat in one direction.

I think that making a fixed skeg would make it a nuisance for beaching, carrying, storing, etc. and making them retractable is quite easy.

So this was a long-winded way of saying "yeah, you probably will need some trial and error to find the best size skeg" :-)