In general, she chooses thoses areas because they’re coastal, and thus would be easier for Ironborn to reinforce and resupply in the event that the Northmen tried to take them back, and easier to get Ironborn to settle since they’re not trying to get fishermen to become fishermen, and because the Ironborn have held them in the past.
As to why those places specifically, Sea Dragon point likely belongs to House Glover (given its proximity to Deepwood Motte) and thus would be land that House Glover could transfer legally to the Ironborn, providing legitimacy to Asha’s conquest. Stony Shore, which the Ironborn occupied during the War of Five Kings, probably doesn’t have any other population than the fishing villages (because of the poor quality of the soil), and thus may indeed have been (temporarily) depopulated by Theon’s raids, which will make it easier for the Ironborn to continue to hold.
In terms of how much space, Asha describes them as “ten times larger than all the isles put together.” I estimate (*WARNING: A HISTORIAN IS DOING MATH*) the Iron Islands is about 16-25,000 square miles, that Sea Dragon Point is about 17,000 square miles, and the Stony Shore is maybe 20,000 square miles, so Asha is exaggerating somewhat, but the overall point isn’t wrong.