How can I calculate number of bits of a decimal number?
Not sure if I understand exactly what you're asking but... assuming there are 8 bits in a byte, and it takes 8 bits to make a "1" "7" "3" or "9" then you would simply multiply 8(bits) x 4(characters) = 32 (bits) With that logic, something like 173912...
can I use an unbroken low port bit to start training my horse?
We've used similar bits on green polo ponies and they've worked quite well.
what would be the best bit for my horse?
You definately want to get away from the chain link mouth peice if your horse is sensitive. I personally don't like wonder bits as it gives to much "bend" to the horse. If your horse can be controlled in a loose ring, use it, but you don't have much ...