I am a commissioner in a 14 team mixed 5x5 root league. We roster 23 guys with a NA and 2 DL spots. Fact of the matter is, if the injury bug hits your team, you should be forced to make some tougher decisions. You shouldn't be able to roster more people for free.
I think you have it good with 2. I would be against moving to 4. Many teams don't use 2, some teams get hit harder. It is a fun variable in the league. More DL spots, means you can get more guys for free, and if they turn out, you got them for nothing. That isn't too fair.