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.
Double the amounts of grain and water. Ensure that your mash tun is also wider, so that your grain bed isn't too tall.
Since you doubled the volume of hot liquor for the mash, you should end up with twice the volume going into the boil. Also, if you follow the rule of keeping your heat such that you lose 10% wort volume per hour, then your volume should exactly double.
There are several formulas to estimate hop utilization, but generally they are a product of the hop weight, alpha acid percentage, and wort gravity. IBUs, volume, and hop utilization are directly related, so if you expect your gravity to be the same as the original recipe, and your volume doubles, then your hop quantities should double as well.
...but this one is tricky, since most homebrewers (and a lot of microbreweries!) don't calculate yeast pitch rates, but instead just toss one vial/pack of yeast into the wort. If you want to be precise, a good guideline is: cells to pitch = (1 million) * (mL of wort) * (°P of the wort)
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.