I have a small condo that is approximately 790 square feet. It is a townhouse with 2 other units on either side. There is fairly good insulation in the attic but not so good in the floor where there is a garage underneath. On one of the open walls is a 12 foot sliding glass door (Anderson relatively new) and one dormer window on the remaining open wall.
I need to replace my heating-AC system which unfortunately is in a small utility "closet".
My current gas heater is 75,000 BTU and when it was in top performance it doesn't cycle much. Is a heater of 50,000 BTU sufficient to heat my condo even in the coldest weather?
What would be the minimum safe and most effricient BTU for a gas furnace in my condo?

Yes 50,000 BTU should run just fine. I believe your furnace now is very oversized. The newer furnaces are also more efficient and you also want to look at the output not the imput when buying.