I have a 20 gallon Concord tri-clad kettle that I've used exclusively for BIAB (over a Blichmann propane burner) for over six years and it has worked quite well for 5 and 10 gallon batches. I've done the occasional 15 gallon batches by incorporating a Concord steamer basket that fits the kettle to hold the bag and then pumping 170F sparge water over the suspended grain. For this, I use something similar to Alton Brown's "turkey derrick" which involves a ladder, some rope and a couple of "carabiner-like devices." This allows me to do a "hanging basket sparge" and thus I can get better efficiencies and makes 15 gallon batches doable despite the Concord kettle being closer to 18.5 gallons.
I just purchased a 22 gallon BrewBuilt kettle for an electric BIAB system. I've only tried a couple of 11 gallon hanging basket sparge brew days, and thus far, that system is working quite well. In fact, one was a Russian Imperial Stout that used 46 pounds of grain for an 11 gallon batch (O.G. about 1.110), really pushing the limits of that system. I have not done a 5 gallon brew day yet, but see no reason why a 5 gallon batch wouldn't work fine.
If you are planning to use propane, I see no reason why a 20 gallon kettle wouldn't work out fine for 15 gallon batches so long as you add some sort of sparge step. If you want to simplify and just do a standard 15 gallon full volume BIAB mash, then you will certainly need a larger kettle. Also keep in mind that if you are planning to use a standard electric element, that will require some sort of steamer basket or BIAB "false bottom" like that sold by BrewHardware to prevent the bag from contacting the element. Apparently, the Blichmann Boil Coil does not require this, but standard water heater elements do. With a water heater element and a false bottom/steamer rack to separate the bag from the element, you might be pushing the ability to mash sufficient grain for higher gravity 5 gallon brews. So, this is something to consider during your system design. This really shouldn't be a problem with a gas fired kettle, especially if it incorporates a tri-clad bottom.