Does anyone know what size a 2018 Diamondback Line bottom bracket is?
A couple things to start. First, are you looking to replace the bottom bracket and keep the same crank set? This is significant because the crank set choice largely guides the bottom bracket type and length of the spindle in a bottom bracket(BB). Thus, if you're looking to mount a different crank set, you'd be looking to the requirements for that. Second, a major factor in determining BB compatibility is the width of the bike's BB shell. A Diamondback Line will have either a 68mm or 73mm BB shell, and you should determine that measurement right off the top as it, too, determines the BB required.
Despite numerous hits from a Google search, the best anyone, including the manufacturer, can say is that the stock crank has alloy arms and a single, 30 tooth, narrow-wide chainring. The specs list only, "cartridge bearing bottom bracket" for that part, and I could not find reference to the BB shell width despite multiple postings on the geometry.
However, one can get enough information from the bike to guide a correct replacement purchase. To obtain a direct replacement cartridge bearing BB, you'll need to know the width of your shell, the length and type of spindle it has and the type of threading the bike's BB shell has to accept the BB. Regarding the spindle, the length is measured end to end WITHOUT including the crank arms in the measurement. This can be a little tricky without removing the cranks. You could remove the crank fixing bolts that thread into the spindle to secure the crank arms. This will give you access to each end of the spindle and from here you can use a tool that measures outside diameters (a spring outside diameter caliper or a long reach digital caliper) or measure the distance from the outside of each crank arm to the end of the spindle of that side, then measure the overall distance from outside to outside of the crank arms and subtract the sum spindle end to outside crank arm measures. You'll then that's the BB spindle length. By type of spindle I'm referring to the shape of the ends where they interface with a crank arm. It will be either a square shape (termed a square taper spindle) or the spindle will have splines (Octalink and ISIS are two possibilities).
Here's what I can say based on experience. A Diamondback aluminum frame will most certainly have a BSA threaded shell (that's 1.37 inch diameter x 24 threads per inch-TPI), and requires the same BSA threading from a BB. Most decent mountain bikes from the late 2010's have a 73mm shell. You likely are dealing with a square taper spindle.