I work with any size block. They all go together in the same quilt quite nicely. Some times it is a challenge to put them together, but it works.

I use what ever size of block will give me the desired result and dimensions. I've used 3" blocks right up to 16" blocks. I don't have a distinct preference.

It depends on what mood I am in.

I like that! Size of blocks depending on mood......Ummmm...food for thought!

I've been doing 12 1/2" blocks, 6 1/2" blocks and now I've started doing 3 1/2" blocks. It just depends on the pattern, and what I'm trying to accomplish.

varies with pattern

After working on Dear Jane and, then, Sylvia's Bridal Sampler, I find 12" blocks gigantic unless there are a lot of pieces. If you want to get finished fast, though, there's nothing like a set of 42 12" blocks!

When I see the word rule, I just turn pale. Do whatever makes your heart sing. In my quilting world there are no rules - suggestions for what may be easier - what may happen if you fail to take a step - but no rules. When someone tries to impose there rules on you remember that the pioneer women who made quilts made them from necessity and they just did what they had to keep their families warm - No rules.
A local instructor had a fit when the student used burgundy thread for redwork on beige fabric. It was beautiful. I quilted it and it was just stunning. Another example of quilt police. I must be a rebel at heart!

I think 12" blocks are the most popular because of the possibilities of units that are divisible by 1-1/2, 2", 3", 4", 6"....no 1/4", 1/8", 1/16" measurements to worry about that you get when you start to do smaller blocks.
The smaller blocks are so cute...and they can be addicting...they take just as long to make as the bigger blocks.
There are so many patterns to chose from in all block sizes...personally, I like 8"-9"-10" blocks the best...12" blocks start to be too big to play with as much...can't get as many in a quilt.

12" blocks are probably the most popular but I like 9" blocks and some people work on 3" blocks...imagine that!!!