The BX25 has an 8 or 12" bucket size, general use the 12" is a good fit, there are many options for trenching from 3rd parties like BXpanded. I use the 12" and a thumb all the time.
Mine also came with the 12" from the local dealer. To be honest, I've been thinking about expanding it to 16" . The machine has enough power with the exception of ripping roots and large rocks.
I bought mine and requested the 8".
While it takes three scoops with the 8 to equal two 12's in a big hole (that can add up) it is also nice to be able to dig a deep and narrow hole too, that was my logic.
Power wise, I have no doubt the 12 is fine.