What he said! 

VG's are used as original equipment on full-size, like some Falcons and Learjets, not so much as 'bandaids' but as elements that do several things to enhance performance with minimum penalty. I can attest to their effectiveness on Cessna twins, the VGs totally transform them and tame some bad characteristics. For a real bandaid design approach, look at the tail of a Beech 1900.  [X(] 

As BM said, 15-20% of wing chord will be right. Height and shape will be up to your skills, as scale VGs will probably not be very effective due to scale effect, Reynolds numbers, etc.  Believe it or not, the aftermarket VGs use double-sided tape exactly like our servo tape for attachment.  You might start out with plastic angle stock from the home improvement store and trim it to your dimensions. You may have noticed that some VGs alternate evenly (facing left of center, then right), and others point toward the center. Just a difference in who designed them, and for which aircraft.  They do work, as many bush pilots will attest.