You need to know the lux/footcandles delivered to the subject from the lamp. The problem with most DIY lamps/bulbs is that they rarely give a nice number in terms of feet/meters vs. fc/lux. This is the value
that factors into your camera f-stop and ISO selection.
Hence the distance from the lamp to the subject is the important value, not 'how big the room may be', other than that constrains the maximum distances that may be involved... well, a 'small ' room does suggest that walls with reflect some light that may affect the light on the subject...so that needs to be accounted for...
