How to figure distance vs. TV screen size?

I want to buy a no-frills TV for the bedroom…c. 8.5–9.5 feet from eyes. Would 42” do it? The 47” seems enormous for the room, in addition to costing more. I also want to be able to read the closed captions occasionally. I suppose I could lie down on the floor at the store. Usage would be mainly for late night falling-asleep or insomnia.