I am working on a project with rechargeable Double A batteries, and the load is 7v so I need 6 batteries (each is 1.2v). I also want the capacity to be as high as possible, so will wiring them in series increase capacity?
If whatever you're powering has regulates the voltage down to a specific voltage with a DC-DC converter, then yes additional batteries in series (beyond what is required to achieve the required voltage) would increase the effective capacity. Otherwise, as said above, adding batteries in parallel would increase the capacity.
I think biggest batteries in 1.2V range are like 2000mAh, which would end up being the capacity for your custom 6 batteries in series (7.2V)
There can also be problems with long term life expectancy on self building bigger things by putting them in series. (due to differing internal resistance of batteries and thus the batteries kind of discharge/charge each other)
You could get around it possibly easier by buying ready made 7.2V battery and charger, they come in many capacities.
If whatever you're powering has regulates the voltage down to a specific voltage with a DC-DC converter, then yes additional batteries in series (beyond what is required to achieve the required voltage) would increase the effective capacity. Otherwise, as said above, adding batteries in parallel would increase the capacity.