The long and short answer to the question is it depends on the grains and what you hope to achieve by steeping them. If you expect some conversion even 2.5 gallons is to much water for a couple lbs of grain. If you are just pulling color and flavor out of the grains, it does not matter how much water you steep in.
Again this varies by the type of grain you are steeping, for example some grains crystal malts for example have no active enzymes in them and lack the ability to convert the complex starches, so in this case you are just pulling out some flavor and some color from the grain. If you are putting in 2 row (stupid I know, but this is an example) there are active enzymes that can breakdown the starches, but if you put 2lbs of 2 row in 2.5 gallons of water it will be so diluted it won't get proper conversion.