Sizing shouldn't be too hard. As for length, the 74" is just about the right length of me. It came out to be just under that in reality but still works well for my 5'10" height. I would say for 6'2" that 78" or 79" should be long enough.
The width is something that a bit of experimenting would help. Size 15? Wow. Taking a guess at that will be tough, but what I would suggest is to stand on a piece of poster or cardboard and draw a circle around your feet that seems like how much space you would want for them. You can then measure this circle. The circumference of that circle is about how wide you want the quilt to be at the foot. I really wish I could give you a more exact number. From a google search your feet are probably about 2" longer than mine, so you would probably want to add somewhere like 4" as a quick guess. On the width, depending on how slim your body type is, I would suggest about the same increase in width. The 50" is VERY generous for me, even sleeping on my side, so maybe 2" or 3" only if you sleep on your back. You could once again do a measurement of your chest, but include your shoulders, to see about what you would need there as well.
The good news is, a quilt 4" wider at the foot and head and 4" longer comes out to be only 22.9 oz theoretically for fabric and insulation. It isn't the lightest option out there, but at a cost of about $80 bucks probably, it is a great cost/weight ratio.