Does the Rainmeter crosshair work in fullscreen?

No. It only works in borderless window or windowed mode, because the overlay is drawn on the desktop above the game window.

Can I get banned for using it?

The author says no: Rainmeter is a desktop customization program with skins, not a hacking tool, and it does not touch the game. Still, follow the rules of any third-party server you play on.

Can I change the size or style of the crosshair?

Not easily. You have to edit the crosshair.png image inside the skin with an image editor such as Photoshop or GIMP.

Binds & Console

KF2 Custom Crosshair Overlay with Rainmeter

This short guide, written in June 2015, shows how to put a custom crosshair on screen in Killing Floor 2 by using the free desktop customization tool Rainmeter. The same overlay works with other games too. The crosshair is an always-on-top picture placed at the center of the screen, so it only works when the game runs in borderless window or windowed mode.

Setup

Download and install Rainmeter from rainmeter.net.
Download the author's crosshair skin and extract it.
Move the crosshair folder into Documents\Rainmeter\Skins.
Right-click the Rainmeter icon in the system tray and choose Refresh all.
Right-click the tray icon again and open Skins, crosshair, then crosshair.ini. The crosshair appears on the desktop.
Right-click the crosshair, open Settings, then Position, and enable Stay topmost. Drag it to the exact center of the screen.
Right-click again, open Settings and enable Click through, so mouse clicks pass through to the game.

FAQ

Can I change the size or style? There is no simple option. Edit crosshair/crosshair.png in Photoshop or GIMP.
Will more crosshairs be added? The author said maybe, if the guide became popular.
Can I be banned? The author says no, since Rainmeter only draws on the desktop.

FAQ

Does the Rainmeter crosshair work in fullscreen?: No. It only works in borderless window or windowed mode, because the overlay is drawn on the desktop above the game window.
Can I get banned for using it?: The author says no: Rainmeter is a desktop customization program with skins, not a hacking tool, and it does not touch the game. Still, follow the rules of any third-party server you play on.
Can I change the size or style of the crosshair?: Not easily. You have to edit the crosshair.png image inside the skin with an image editor such as Photoshop or GIMP.