Hope you understand Bill was just kidding with you. The mental image of starting a diesel
with a D size flashlight battery is a bit amusing. And you weren't exactly wrong, just left out a small detail. Large batteries can be D size but with a number in front, like 2D, 4D, 8D larger numbers = larger battery. I'm guessing you have a 4D or maybe 8D.
A couple of things to clarify for you. First your starter most likely needs 12V to operate. Yes you can certainly use 6V batteries but you need two connected in series so the voltage adds up to 12V.
If your current
system has D size 6V batteries they are probably what are called "Deep Cycle" batteries. As noted in several comments, deep cycle batteries will certain start an engine but getting a "Starter" type battery would be better. A starter type battery will be:
3. more efficient
4. overall be better for starting than big 6V, deep cycle batteries.
You just need to insure two things for either battery type:
1. The starter battery is isolated from the house loads like lights and such so it is not accidentally discharged and not available for starting.
2. If the starter battery is isolated from the rest of your battery system you will need to make sure that this battery is getting charged when the engine is running or the battery charger
or other sources are operating.
Both of these are fairly easy to set up and are discussed in other threads on the forum. If you need to sort this out and can't find the appropriate discussions just mention it and I'm sure someone can find them and post the link.