With FPDF the page size is defined when you create the PDF prior to starting to fill it with content. If you don't specify the size then the default page size of A4 is used.
You would want to allow left and right margins so that everything is inside the area that prionters can actually print so you'd deduct those off the total width and then divide the remainder by the number of columns you want.
If you didn't override the default then the base measurements are millimetres. An A4 sheet is 210mm wide so if you allow 10mm left and right margins (which is the default) then you have 190mm to split between your columns.
Note that I don't have all those default sizes memorised, I just looked them up quickly in the FPDF documentation while I was writing this reply.