Its 2 miles wide. A blink of an eye in a jet.
Basically, the Turks have been targeting the Russian jet route for a number of days. they notice that the Russian jets cut through a 2 mile wide spit that intrudes several miles into Syrian airspace such that the Russian pilots.
Russian planes spend about 15-20 seconds overflying the 2 mile wide spit of Turk real estate that intrudes into the Syrian border.
The flight of the F-16 air to air missiles takes longer than 20 seconds to launch and travel to the target.
So either the Turks fired at the Russians while they were in Syrian airspace to allow the missiles to hit the Russian planes when they were in Turkish airspace, or they launched on the Russians while the Russians were briefly flying over the Turkish spit and were technically in Turkish airspace, but if they did this it's very likely , almost 100% certain, that the missiles impacted in Syrian airspace.
Regardless, if reports are true, then the Turks gamed this shoot down to try to draw NATO into the fight against Russia.
FWIW, Turkish jets violate Syrian airspace on a regular basis to provide air cover for ISIS