So is it the three bolts that hold the blade on is what you are asking??? Do you want the size of the bolt itself?? Or just the size of the wrench to fit it??
There are two different types of 54" BX1860 decks, and they use different spindles.
Nevertheless, a replacement blade bolt for either is not available at a hardware store, due to the style of the shoulder on the bolt and the grade of the steel used for the bolt. Ergo, your dealer or online (example: Messicks) is the place to get the replacement blade bolt.
If you are looking for the size of the socket for the bolt head itself, you should specify that is what in fact you are asking for. In any case you need a 1/2" drive 6pt socket, and either a long breaker bar -- or an electric or pneumatic driver.
ps
At this rate I predict it will be at least three days before we get enough useful info to help further.
According to messicks, the bolts are 12mm with a 1.25mm pitch. No reference to style or length though.
If wrench size is what is in question, one website I looked at said it could either be a 17mm, 18mm, or a 19mm. Just depends on wether its ANSI, DIN, or JIS standard.
Either way, those are all sizes that come in a 1/2" drive metric socket set.
Well then its most likely a 17mm, 18mm, or a 19mm. All of them usually come in a set. Do you have any tools at all or are you just wanting to purchase a single socket and ratchet??
If it is cause you perhaps only have standard sockets and nothing metric, if it is a 19mm, a 3/4" should work, if it is a 17mm, an 11/16 should fit but be a little snugger than normal. If it is bigger than the 11/16 and smaller than the 3/4, then you know its an 18.