Hi Shannon!
I am looking for the Yamaha Pianica P-37D. The white key is 93.9 mm long,12.2 mm wide, the black key is 59.6mm long, 8.1 mm wide. If I’m not mistaken, this is the same instrument Daren used to model his 3D Melodica after.
I’m also planning on 6 rows of keys but 4 would do it as a minimum.