Are snowboard boots too big if they are 5cm bigger than the board's width?

Do up the boots and strap them in the bindings without you in them. Check the boot. Make sure the boot is equal over the board on it's heel and toe. Now check if you will have heel or toe drag. With the boots still in, lift the board as if you were carving on your heel/toe side. If the boot hits the surface with you only lifting the board a little, you need to get a wider board, or squeeze into smaller boots.