Essentially the same size as a weather balloon like the one above.
Gas released from a pressurized tank can be freezing cold. I wonder if that would cause the gas in a recently filled weather balloon to be markedly colder than the surrounding atmosphere, like Go Fast. In a quick search I didn't find such a weather balloon imaged on FLIR.
You can't see if it's hotter or colder than the surrounding atmosphere as surrounding atmosphere does not show up on IR. The blob here is showing up as cooler than the ocean.
It's cooler because it's higher, in cold air, and any source of heat is either hidden, insulated, or there is no source of heat. This could happed in different ways:
1) Helium Balloon - no source of heat, so it's the same temp as the air at 13,000 feet.
2) Bird - internal heat, but possibly insulated and masked by feathers which are cold, as they are in cold air
3) Small plane - Engine heat, cabin heat, and exhaust heat. Unlikley to show up as cooler than the Ocean, but perhaps possible at a distance with the high wing hiding most of the heat sources, so on average it's cooler.
I'd lean towards that order for probability. I'm torn between bird and balloon as a balloon fits the temperature profile better, and the shape. But there's a lot more birds than balloons.