LITS (Lasse's Ideal Team Size) is between 2 and 4 developers. Now, that's mostly an ideal size based on a pure team dynamics perspective and doesn't consider the properties of a specific context. In practice, the LOTS (Lasse's Optimal Team Size) might be larger than 4 developers due to specialization of skills, desired development speed, etc.
<span style="font-size: smallest;">Yes, I just made up those acronyms. Yes, I thought it was funny. Yes, I did sleep only 4 hours last night, half an hour of which took place in an airplane.</span>
I'm not even sure the phrase "ideal scrum team size" has any reasonable meaning. I guess it might help if you told us why you are asking...
Most teams that got larger than 12 seem to prefer to split up and practice something that is often called a "Scrum of Scrums".
Regarding "failure stories", most Scrum teams notice after a while that project management isn't enough, that they also need some practices that actually help them *build* something. Many of them seem then to start to adopt XP practices.
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
In a presentation linked from another post around here, Ken Schwaber talked about Scrum failures that come about because Scrum is too darned honest and transparent. If you have an organization that likes to lie to itself - "Yes, we're 80% done with requirements analysis and totally on schedule!" - then that organization will hate Scrum. There is no place to hide bad news and the impact of any change - eg scope creep - is unavoidably obvious.
It's also too much work for some customers. It makes the customer responsible for success by managing scope and time to a conclusion that has value. Some customers would rather say "Here are the requirements documents. Call me back in 18 months when you've completed all of them." Won't happen in Scrum.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi