I found a recipe that I would like to try, but it is for a 5 gallon batch and I want to make a 10 gallon batch. Is there anything special that I need to keep in mind or calculate when doubling a recipe or is it a straight up x2 conversion?
I mostly agree with chrislarson, but I would propose a few differences, if you wanted to be more precise.
If you thought your equipment setup was going to result in a higher evaporation rate in the boil, like chrislarson, then you would need more water (either in the mash or added later) and consequently, less hops.
Mash: Just double the amounts of grain and water.
Boil: You'll need 250% of the water to account for more evaporation (In my experience).
Hops: Judging by what BeerSmith tells me when I scale a recipe, you'll need about 175% of the hops by weight. For both bittering and aromatic.
Yeast: A 16 oz (.5L) starter has always been enough for me. Even with high gravity beers.