The good news is when doing full boils everything goes in you favour.
Most online recipes are done for full boils, You can use the same recipes the the "Experts" use, You will have the facility to steep, mini mash and go all grain with the same pot, your hop utilisation will be better so you can use less hops. You have less chance of malt caramilsation, you can make lighter beers.
Any way the answer to you question is that you do not need to scale up your recipes. The only thing to take not of is that you don't need to reduce the volume of the wort as AG brewers do. We need to collect all the sugar from the grain and often end up with a thinner larger volume of wort.
All you need to consider is that to get goo hop utilisation and the correct bittering you'll need to boil for 60 minutes so you'll need 5 gallons plus what ever evaporates. You don't need to worry about a vigorous boil because the extract will already of achieved an hot break. So were as AG brewers may need to reduce 7 gallon to 5 gallon for the previously stated reasons you may well be able to get away with 6 gallons to 5 gallons.
I don't do extract full boils so maybe some one who does can give you details of thier volumes and procedures.