I use a 25 litre cooler, and it is fine for a grain bill up to about 5.5 kg, for a 23 litre batch.
Anymore than that, and it is a struggle.
I use a manifold, but batch sparge, with 2 sparges after the initial runoff.
Mash efficiency is usually around 90+%, so it works fine.
A couple of times I tried to do a weizenbock with 7 kg of grains and an estimated OG of around 1.070, but I ended up with a horrible stuck mash, having to split the mash into two lots to extract the wort, and my efficiency suffered horribly.