On a prior thread, we got into an argument over realistic ranges for ship to ship space combat with lasers.
I had thought a couple of light seconds (a light second is 299792458 meters)
Someone else argued that lasers would be limited to point blank range due to dispersion.
Dispersion of a laser is measured by the Rayleigh length which is the distance over which the beam diameter will double and is calculated as(PI*(initial beam diameter^2))/wavelength. https://en.wikipedia.org/wiki/Rayleigh_length
I decided to calculate Rayleigh length for a few options. I found a list of naval cannon sizes here: https://en.wikipedia.org/wiki/List_of_naval_guns_by_caliber
The smallest naval gun in the US arsenal (WWII) is 28mm.
The largest is 406mm (WWII 16 inch gun)
The most modern US naval gun listed is 155 mm
So I will use those three options for beam diameter.
I also decided to check three wave length options, since Rayleigh length is inversely proportional to wavelength.
I went with Hard X ray (10^-10 meters), extreme UV (10^-8 meters)
Visible light (10^-6 meters)
xray = 24,630 kilometers
UV = 246 kilometers
VL = 2.46 kilometers
xray 2.52 light seconds
UV 7547 Kilometers
VL 75.5 Kilometers
xray 17.27 Light seconds
UV 51,785 Kilometers
VL 517 Kiliometers
Any of those size beams is still going to be able to put all the initial output energy into a Ship sized target at the Rayleigh length.