Hi I recently started riding again. My son has got down and always wants to go for a ride. I'm 6'4 what size bike should I use and how do I figure my current bike size.

You decide what bike you want to buy and then get the size that the manufacturer suggests. Some only go by height, but many go by height and leg length, maybe arm length too. However different things might make a size or two one way or the other feel better. That's something we can't tell you based on what you've given. 76". And just because you are say a 60 cm in one model, doesn't mean you'll be a 60 cm in another.

To me, IMO, all these fit calculators out on the web aren't worth anything unless you are having a frame custom made for you. And even then, you should work with the frame builder to decide the fit.

I've ridden everything from 64 cm to 56 cm and been very happy on all of them. I'm only 71"

Very tall and very short people always have more trouble getting a good fitting bike, simply because bike manufacturers are focused on the middle-sized folks 'cause that's where the profits are. You might have to look around a good bit to find a shop which carries a size which will fit you. Be all that as it may, my view is that more information is better than less information. There's a very good bike fit calculator out there: https://www.competitivecyclist.com/S...ulatorBike.jsp

Get someone to help you measure yourself, fill in the blanks, get the numbers, and then more than anything, go by effective top tube length for picking a frame size. You can go online to manufacturers' websites and look at the frame specifications to see what might fit you best.