I have a 5 gal Rubbermaid and for the most part it works just fine. I got it because it was $17. I am concidering getting a bigger one but it is not on the top of my list. I have missed temps and not been able to compensate much because of space, but once I got the temps figured out all has been great. If you can get a 10 gal for not much more I guess I would do it.
I have the 5gal drink cooler MLT. I can fit 12.5# of grain and batch sparge. I usually aim for under 12# in my recipes because it gets really tight with anything larger.
I am basically draining a full cooler 3 times to get my pre-boil volume. I typically get 78-82% efficiency.