Well I disagree with zen on this one....if you don;t have the means to chill a larger amount rapidly (under say 45 minutes) you are better off boiling the largest amount you can reasonably handle and chill to pitching temp, In my case before I bought a chiller that I could use both on the stove and on my turkey fryer kettle I could get 2.5 gallons of wort from boiling to high 70's in a decent amount of time.
I could get it down to that temp in about 1/2 hour in my sink using 2 bags of ice and water.
As good as a larger volume boil is for extract batches (for all batches actually) you don't want to be sitting at 90 minutes and your wort is still in the 120 degree range.
Plus do you know if your stove and pot can handle larger volumes of liquid going from cool to boil and maintaining boil for an hour? On my electric stove it usually takes nearly an hour to get 4 gallons of wort to near boiling as it is.
If this is your first batch I would follow the boil amounts called for in the BB recipe...the recipe is balanced out in terms of hop utilization and things like that, and rather than adding another variable and strying too far off recipe (You will have enough to "worry" about) those recipes are nearly full proof.
and BB's are NOT pre-hopped kits like zen assumes.
This is the first I assume of many brewing adventures...you don't have to go off the reservation whole hog and follow all the suggestions you find on here the first time....just the onese involving patience, relaxing, not relying on an airlock and using your hydrometer.
Once you address the chilling and boiling issues then you can go to bgger boils.
To avoid scorching on this batch, take the pot off the heat when you are adding the extract and stir stir stir to keep the extract from falling to the bottom and scorching.
Have fun