I have built a few high-wing trainer types with spoilers, based on numbers in Andy Lennon's book, and had good results, using the general guidlines (for a "plank" wing) of:

Spoiler location: at 70% or more from the leading edge of the wing
Spoiler chord : 10 to 15% of the chord
Spoiler span : 50% of the semi-span
Deflection : Depending on useage for roll or as brakes/lift dumping


There's more info on ailerons in NACA report 605, at http://naca.larc.nasa.gov/reports/1937/naca-report-605/

Andy also points out that, with slotted flaps depoloyed, the spoiler is much more effective, as it opens the slot lip further, and dumps a lot of the flap's lift.... functioning as a "slot-lip aileron". It works great on a model too.