If the organization has successfully adopted agile practices, then any project is a candidate
In this case its a matter of taking the “next step” in terms of size and complexity. It may mean taking on a longer, more important project, or it may mean introducing additional dimensions of complexity such as distributed teams or multiple teams.
If the organization is just starting out and is looking for a suitable pilot project, then small low risk projects would be better
This is because the emphasis will be on learning what works and what doesn’t work in the context of the organization. The point to keep in mind is that when a company is transitioning to Agile, its not just learning the process, its learning how to deliver. Part of that is understanding where the sharp edges are in the organization and where change needs to happen.
Lastly, to help with a decision matrix, seek the expertise of a good coach.