Welcome to the torment of trying to choose a projector screen.
A lot of us discovered once we got into front projection that choosing the exact screen material and screen size could be even harder than the decision on what projector to buy, for many of the reasons you are finding out.
You are on the right track: projecting images on to your wall to investigate the image size you want is pretty much the best you can do in making this decision. And everyone is different in what they prefer. But of course it's not only about image size preference; you have to consider other technical criteria such how bright your projector is and how bright an image you want.
You're right - the bigger screen will tend to require more light horsepower, which can have consequences over time as bulbs dim. And that brings in questions of what screen material to buy. If you are going for a larger screen
then you may have to go with a screen with some gain. But if you have a challenging room for projection, e.g if you have bright decor that might reflect light back to the screen and wash it out, you may wish to go with a "speciality screen" that is darker...but if you do that you may not be able to go for the 119" screen as it would be too dim.
So, lots of things to think about.
But sticking with the concept of sheer image size and viewing comfort, it seems you have just begun testing this issue at your home. I'd advise you to keep testing a while longer. Why? Because we tend to adapt to different image sizes over time. When I started projecting images on my wall, from around an 11 foot viewing distance, I felt that a 92" diagonal 16:9 screen would be as big as I needed. Compared to my TV it just felt so huge and cinematic. But the planning for my home theater took quite a while and during that time I continued to test watching movies at various image sizes.
I found that I adjusted pretty fast to larger image sizes. I was projecting bigger and bigger and bigger images. And once I viewed a movie at, say, 110" diagonal, going back to a 92" diagonal, or even 100" diagonal image felt small and less cinematic. More like a "big plasma" as you put it.
I even enjoyed CinemaScope movies at up to 10 feet wide. Here's a post I made in my home theater build thread about how much indecision I went through when trying to mark which image size I wanted:http://www.avsforum.com/avs-vb/showt...5#post18733105
And as you can see from that post I ended up going to a fairly unique Variable Image Size set up, where I just vary the image size to my whim.
Having lived with this system and having experienced my choice of image sizes I can say a few things: I often view 16:9 images at between 102" diagonal and 110" diagonal (although for some occasions much bigger).
I find 105" diagonal a pretty good sweet spot for image size and image vividness in terms of sharpness and brightness.
However, to get that real "cinema" effect I find moving up to 110" to be significant. From an 11 foot viewing distance a 110" diagonal 16:9 screen image seems to be a sort of "sweet spot" image size in terms of feeling large enough to enter that "Cinema" feel, beyond a giant plasma vibe, yet retaining an excellent sense of brightness, image density and sharpness, so you still get a "holy cow" level of clarity from a great source.
I've used that size numerous times and neither I nor a single guest has ever felt overwhelmed at all. We had a bunch of guests to watch Toy Story 3 at that size and it just felt great.
Now, you are talking about sitting at 15 feet and that's a significantly greater distance, reducing immersion in the image. You may want to try that image size while sitting closer for a movie or two, to see if you adjust.
If I didn't have the choice of different image sizes and had to choose a 16:9 screen of only one size, I have a feeling I'd go for 110" from my 11 foot distance, since it's a good sweet spot for a "wow" feeling for watching movies and also because 2:35:1 movies wouldn't be as compromised in impact as they would at a smaller size.
ETA: When I say I like the 110" screen size I tend to be thinking of Hi-Def/Blu Ray movies as a source. There are now so many movies available on Blu Ray that it's hard for me to even find time to watch the few DVDs I have left. However, if you happen to view a lot of DVD movies you may want to take those source limitations into account, in terms of screen size. (But then again...HD/Blu-Ray
is the future for quality movie sources, so plan for that as well).
FWIW. Best of luck.