These days the amount of emitted light is dictated more by the bulb not the size of the can. There are now 4" LED's that will put out 950 Lumens or more. 6" cans look dated and there is not much choice in alternative bulbs. Go with LED so you won't have to change out burnt bulbs every couple of years, especially with higher ceilings. Color temp is the next consideration, traditional soft white light is in the mid/lower 3,000 K range. If you wish a more daylight color then select 4,000 K or more. Just make sure you like that whiter daylight light, some clients find it to be a bit institutional looking.
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in stairwell
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