Does this soldering gun come with batteries?
No, it doesnt. You need to buy your own batteries, the usual Dewalt 18V/20V lithium-ion batteries can be powered, such as DCB201 DCB204 and so on.
What is the maximum temperature that the cordless soldering gun can reach? Does it heat up fast?
It can reach 842℉ in about 30 seconds, which is 30W of power can greatly improve my work efficiency.
What size of wire can I use with the battery soldering iron?
0.8 mm - 1.2 mm in diameter. The package comes with a roll of tinfoil, which I think is enough for everyday use.