Screen size is largely personal preference. Room treatment is a must for best experience, and things like tile may sound good, but there is a reason that theaters use dark carpet. It works best for both audio and video reflections.
But size is not only personal, it shouldn't be decided upon too far ahead of time if you can avoid it. Instead, buy the projector of choice, then try some different sizes out to see what you prefer.
I know I like about 10" of diagonal for each foot of viewing distance. So, I sit about 16' away from a 161" diagonal screen. Some people really prefer closer to 12" of diagonal for each foot of viewing. Some prefer more, some less. But, I think the golden rule if you must decide ahead of time, is to plan for about 10" of diagonal for each foot of viewing distance. If using multiple rows, either compromise between rows, or pick a seat and design around that as a primary location.
Definitely consider wall color as well as ceiling color.
I did this writeup as well which you may find informative...
Getting the right screen size and seating distance for a home theater involves math, some of rule-of-thumb, and a bit of personal taste.
www.projectorcentral.com