I haven't done partial mashes in the way you do, but I brew all-grain and I mash about 12lb of grain in my 5-gallon cooler (which is 20 quarts) - and that's including the whole mash, whereas the colander should only need to hold mostly grain as the liquid drains through... So offhand, I would guess that an 8-quart colander would be plenty big for lautering a mash of 5lb of grain.
Not sure if you've thought about it or not, but doing partial mashes in a small cooler is pretty easy and works great... I used to use a 2-gallon round rubbermaid drink cooler for doing partial mashes of about 3-4 pounds (with room to spare), and it held temps quite well and was pretty cheap and easy to build. As a bonus, when I switched to all-grain, I bought the 5-gallon round rubbermaid cooler and my fittings swapped straight into it as it uses the same spigot assembly as the smaller one did.