Is 50 inch 4k t.v too small if i am sitting 8 to 9 feet away?

For a 20 degree angle, tv manufacturers recommend the diagonal measure of the tv times 2.5 so 50 inch * 2.5 = 125 inch or about 10.5 feet so you'll be fine. Others recommend diagonal distance * 1.5 so about 6.25 feet.

The bigger the angle the closer you need to sit or the bigger the tv you need.

I suggest you go to the store and look for the tv that you want. Then stand 8 or 9 ft away from it and watch the tv to see how you like it.

I personally would go at least 55" because if the screen is too small you're not going to see that much of a difference.

The THX recommended distance for a 50 inch TV is 5.6 feet. Max of 7.9 feet so Yes.