I have 6'5" by 36" closet that I want to build a half bath in Sink and toilet only. I have plumbing from a laundry room already in place will this be enough room to do what I want?

Your length is fine but that width makes it tight. Sketch it out and show it to your local inspector to see if he'll pass it. How will you handle the 3" drain from the toilet and venting up to the roof.