You're getting good help from the members that have stopped by. Remember one thing though, the "rules" on size and height being discussed all come back to personal preference. For example, when you go out to the movies, some folks like to sit as close to the front as they can. Others prefer the middle or back of the theater. Nothing wrong with any of that, just remember that everybody has personal viewing preferences, and yours should shape how your room is designed.
A friend of mine that works in the industry has an incredibly expensive FP theater room, and uses an 80" screen with near field sound. That's his preference. My preference is decidedly bigger, but not huge. There are so many considerations that go into what's right in any given set-up, for any particular person. Fine detail in the image is important to me. I don't want a huge screen for a number of reasons, but that's one of them.
Some of the other variables that may come into the equation is your size (and that of your better half), your acuity of vision, the geometry of your furniture, whether you will recline, what kind and size of speakers you'll use, whether you want to leave room for curtains or treatments or other decorations, and on from there.
Right now go splurge on a roll of 2" Blue Painter's Tape. Bring that and a pencil and a tape measure into the room. Sit in about the space, and at about the height, and at about the viewing angle that you anticipate using, and then look towards where the screen will hang. Determine what seems to be the center-point of a comfortable viewing angle for you. Then play around with screen size. Start with the 110" screen you first mentioned. Live with that size taped on the wall for a day or two. Then go up or down in size as you like. Adjust the "screen" up or down in height so you can see the alternatives in space. Check out what it looks like to have the image really close to the floor if you're considering that.
You'll learn a ton about your project, your room, and what you personally want. It gets way easier to envision viewing angles, and the real space available for curtains, speakers, whatever else once you add a representation of the screen. Obviously you plan to hang your screen 3 feet off the wall, but you can make allowances for that when you install for "actual" screen position.
Take your time and have a blast. The success of your project turns on you doing all your homework now.