Any suggestions on a good large room portable heater?

You don't need to heat 20x15 .  You need to heat where you are located. TV or stereo or other couch does not need the heat if no one sits there, so really you need a directional heater (electric) like a ceramic heater to do your corner.  Most of my furniture is not sat on or nearby. I have my favorite chair where I watch TV from or am on the computer.  The dog does not care as he is always wearing a fur coat.  My space of operations is 3x4 at most.

Logic: is so SPOCK

Try one of these oil filled radiant heaters.  Works well for a large room even though it doesn't have a fan.  I use one in my bedroom that is 18 x 25

The heater in the link from Home Depot will be ample for your room. It needs a small propane tank with it of course. No doubt the store will supply or advise.

https://www.homedepot.com/p/Mr-Heater-Buddy-FLEX-1...