Be careful changing up the boil amount without adjusting hop additions...
The hop additions in the recipe are based on the 2.0gal boil so you'll reach the target IBU for the recipe by following the recipe. If you boil more water (lower gravity of the boil) the hop utilization will be better, thus you will have a higher IBU result.
Maybe you want that, maybe you don't...
Some brewing software (I recommend BeerSmith for a one-time cost of 19.95) would be helpful in adjusting the recipe if you want to do a full boil. A full boil is a great idea, but changing it (boil amount) without changing the hop additions could have an adverse effect in some cases.
EXAMPLE:
Recipe:
7.5 lb. Liquid Extract for a 5 Gallon Recipe = OG of ~1.055
1oz. Chinook 12%AA Hops for 60 Mins
1oz. Cascade Hops 5.5%AA for 30 Mins
.5oz Cascard Hops 5.5%AA for 5 Mins
This recipe, if done full boil (5.5 gallons taking into consideration boil off), would result in ~58 IBU of bitterness due to the good utilization rate of the hops (per Beersmith software).
The recipe if done 4 gallon oil would have some reduced IBU level. I didn't calculate for this volume, but I'm guessing ~40IBU.
If you reduce the boil for the exact same recipe to 2.0 gallons and use 3 gallons top off water at the end, you will achieve ~20-22 IBU of bitterness due to the lesser utilization (more extract/sugars/etc in less water = less room for the bitter hop oils to absorb into the wort) of the hops in the smaller boil.
You will almost definitely notice a difference between 20 and 58 IBU brews...
Simple answer to your original question is to follow the recipe exactly (boil size and hop additions especially) unless you have brewing software (or do a manual recalc of IBU) to correct the recipe appropriately.
Hope this helps.