It’s the “nozzle” diameter – not the filament diameter.
To check, go into Cura Preferences -> Printers -> pick the printer -> Machine Settings
From there you’ll see a window with two tabs… one is “Printer”, the other is “Hot End” and dual extruder heads have two “Hot End” tabs so you’d need to check each of them.
That tab will have the “Nozzle” diameter. Make sure the nozzle diameter defined matches the nozzle diameter of the toolhead.
If using Cura LulzBot Edition, when you add a machine definition – it asks for both the machine and the print-head. It will define everything correctly based on that head. If you swap heads but don’t update the machine define (or just create a new definition for that alternate head) then you can get wonky print results.
The area of a circle doubles each time it’s size changes by a factor of roughly 1.4x (the actual number is the square root of 2, but 1.4x is close enough). E.g. a .5mm nozzle has double the area vs. a .35mm nozzle and 4x more area than a .25mm nozzle. This means that if the nozzle diameter in Cura is set to a value larger than the actual physical nozzle diameter, you’ll end up with over-extrusion.
You can have first-layer issues based on z-offset value. E.g. if the nozzle is too close to the build plate it can squish the filament fairly wide and end up with a poor quality result. This would happen even if all other settings (e-steps, nozzle diameters, etc.) were all correct. If you post a photo of what your printer is doing, the community might be able to take some guesses as to what is happening.