I've 6'3 and I ride a 21". I know there are lot's of frames that are available in 19, 19.25, 19.5, etc. I would go with one of those. Anything smaller is pretty much going to be flatland specific. Look at something like the Federal Hamilton. Steven Hamilton (although crazy and hopped up on drugs) rips and he's tiny.
from what i recall you would be best off with a frame with a 18.5-19.5 inch top tube. no need for 18 inch wheels or anything that extreme. the geometry of the bike can make it feel smaller/bigger. of course your frame choices are now limited because most bmx frames are 20"+ top tubes. but look around danscomp.com or something to narrow your search.