Steeping grains -- how rigid is the volume?

I just wanted to ask for clarification regarding steeping grains and their volume.

In a lot of places (on the net, recipe kits etc), I've read that specialty grains "should" be steeped at 155F for 30mins. Usually, said grains are steeped in a 2 gallon volume which is then transfered to the kettle, and subsequent water+DME/LME added to get to target pre-boil volume and SG.

My question is this: is there any particular reason why this magical number of 2 gallons?

If I am aiming for a pre-boil volume of 3.5gallons, is there anything stopping me from doing my steep in a larger (e.g. 4) volume rather than 2 gallon, thus negating me having to add water to the kettle at a later stage? Is there some kind of theoretical ideal ratio of specialty grains to water? I know with AG there's a working range to do the mash in, but does this apply to steeping too?

Maybe.

If you're using ONLY crystal malt and other already converted malts, you can use whatever volume you choose, probably without any negative consequences.

However, if you've got any other malts like Munich malt, victory malt, two-row, etc, you really want to still with the 1.5 quarts per pound of grain so that you can convert the malts, avoid a too-high mash pH, and avoid tannin extraction.

In my mind, it's just easier to always plan on using 1.5 quarts of water per pounds of grains at 150-155 and you're always 100% safe.

Thanks Yooper, the only malts I will be steeping are:

0.4 lb Caramel/Crystal Malt - 60L
0.4 lb Chocolate Malt (350.0 SRM)
0.25 lb Roasted Barley (300.0 SRM)

That totals 1lb specialty grains to be steeped. With your calculation of 1.5qt per pound, then I would only need to steep in 1qt water....which is way below the recommended 2 gallons.

SO...would there really be any difference if I steeped in 2 gallons or 4 gallons, or will I be extracting undesirable flavours with the larger volume?

Nope, you'll be fine with those grains. Steep in 4 gallons of water.