About this long.
The fight scene needs to be long enough to get the events across. Anything beyond that is just padding, and can be cut. You can easily play an entire fight scene in a paragraph or two and be done with it.
The more complex the scene is, the longer it can run. But, for a simple setup (like two characters struggling over a gun) you can knock that out in a paragraph and then move on with your story.
Fight scenes are not created equal. It may actually be useful to think of it as a game. This character does this, so that character responds, so this character reacts, so that character would react, but they don’t have any options left. A fight scene running on that structure can run for pages, until your characters are out of options and cornered.
For a simple setup, you don’t need a long or elaborate fight scene, you just need to write the scene as you see it. If you can do that in a couple sentences, you’re golden.
There’s an easy mistake to be made here. On screen, or in comics, the violence is the payoff. You watch the fight play out, and that’s its own reward. In text, violence is a vent for drama. The meat of your story is what happens around it. In text, it is much harder to tell a compelling story where the violence itself is the payoff.
You don’t need to approach fight scenes like there’s a minimum word count. Write what happens, as clear and concisely as you can.