I have a battery powered device that accepts 4 x D cell batteries, it also has a 6V DC input slot to run off a charger/mains. There is no mention on the device/manufacturers website of the amperage for the charger/mains.
What sort of amperage would 4 x D cell batteries produce?
D-cells can provide such a wide amperage range (the more the amps, the shorter the run-time) that it's not useful as a guide for sizing an AC adapter.
The equipment should state the amps required. Possibly on the inlet on the device, but definitely on the AC adapter which they sell for the purpose of feeding that inlet. You don't need to buy the adapter, just find a high enough resolution picture where the amps or watts figure can be read. Make sure to read the spec on the DC side. If you read the AC side watts by mistake, they will be a bit higher, and that won't really hurt your project - a few extra watts on the supply never hurts. However, if you read the AC side amps, it will understate amps by a factor of 10 or 20, and that would lead you to get much too small an adapter!
Good alkaline D cells can deliver a few amps for a little while*. You're probably better off estimating the drain of the device (which is almost certainly less) than going from what a D cell could do when stressed.
* I use them as model airplane glow plug ignitors. In typical usage, a pair of D-cells in parallel will deliver 500 to 1000 starts before you need to chuck them and build a new pack.
Somewhere between 8 to 18 Ah, for non-rechargeables. https://en.wikipedia.org/wiki/D_battery
