|Posted: 2011-September-25 at 11:58pm | IP Logged
Most papers I've seen agree that Jupiter is about as large as a "cold" planet can get, diameter-wise. Most wouldn't be more than 10 to 20 percent larger.
On the other hand, a large brown dwarf can take a long time to cool, and will stay "puffed up" for considerably longer than a smaller BD or jovian. Also, a gas planet orbiting close enough to be tidally heated is likely to be larger than a cold one.
In general, a brown dwarf will shrink to the size of Jupiter within a couple billion years, even if it's still hot enough to shine feebly. The slowest to shrink, oddly, are those about 20 J'-masses, as they have less gravity. They'll likely end up being the largest in diameter. On the other hand, by 3 to 4 billion years, a 70 J-mass BD will be smaller than Saturn!
This page, from Dr. Adam Burrows,
has all kinds of data, but interpreting it can be a chore. It's all numbers, in scientific notation! Most important, the radius is in GigaCentimeters!?
He used to have a Brown Dwarf calculator, which was much handier, but when he moved to Princeton, they wouldn't let him run it there, for whatever reason.
For artistic purposes, it doesn't really matter, unless you want the BD to be at a particular distance. The Teff is in Kelvins, and would be more useful.