This is an easy one - the spreadsheet that I linked to and the more recent version that Jimmysuperlative linked to - will tell you exactly what volumes and pot sizes you will need for various sized batches.
The brewers friend link is overly pessimistic about the sized pot you need - Use the spreadsheet - to need
a 15G (60L) pot for 19L (5.0G) of packaged
beer, you would be looking at a brew using 22kg (48.5lb) of grain. And thats gonna be a pretty damn big beer!! Way more than 1.1 OG at the worst imaginable efficiency.
Basically though - you can use a rule of thumb. If you want to do the simple, no sparge, nice and easy version of BIAB - then the minimum pot size you will need to be able to brew a wide range of beers and gravities, will be a bit more than twice your desired fermenter volume.
So you want to brew 2.5G batches - then you will need a 5G pot. 5G batches - 10G pot. etc etc
You will still have a little trouble trying to brew bigger beers and I suggest a bigger pot than the minimum, but for most beers that rule of thumb is fairly OK. Mind you - I think that the 15G pot is in fact the perfect
size for a 5G brewer. It give you lots of space and room to brew anything you want & as a bonus, its big enough to squeeze a 10G batch out of without having to much about with high gravity brewing etc. But its the optimum size IMHO, certainly not the minimum.
So - twice as big as the amount of wort you want to put in your fermenter and you will be OK, but about 50% bigger than that is optimum.
I've brewed dozens of BIAB brews - in everything from 1G to 15G batch sizes. This is my opinion based on the things I have learned doing those brews.
Oh - and for all things BIAB. Check out the New BIABrewer.info forum. Its been set up by the estimable Pistol Patch. Who is not only the principle (he would get mad if I said only) inventor and promoter of BIAB, but has worked his arse off to make sure that there are resources and information all over the place for new BIAB (and AG in general) brewers. If you happen to be in Australia in October.. you can catch him at the Australian National Homebrewers Conference where he is presenting on the topic of BIAB.
In the meantime check out his forum and you should find the answers to pretty much all of your BIAB questions.http://www.biabrewer.info/
PS - Pistol and I don't agree with everything to do with BIAB... where there are discrepancies you can assume I am right and he is a silly bugger who drinks too much and doesn't sleep enough. Or perhaps visa versa.. you decide.