What Size Brush Mower Do I Need?

The general rule of thumb is to allow 4 horsepower for every one foot of implement. Your Yanmar YM2000 has 24 engine horsepower and 20PTO horsepower. A good size mower based on that horsepower would be 5 foot (5 foot x 4hp= 20hp required).

One big consideration is the weight of the mower and, even more importantly, how far that weight is positioned from the pivot point of the lift arms. Grooming mowers (AKA finishing mowers) have several smaller blades arranged in a row and are generally a shorter distance from the rear of the tractor. Brush mowers, however, typically have one big blade so they are usually extending much further back. That weight being a greater distance from the pivot point creates a much monger lever and can cause the tractor to do a 'wheelie'.

That answer got kind of long winded but the short answer: A light duty 5' brush mower is usually the correct size mower for that tractor. :)