I have had a lot of inquires lately about how to lay out your recessed cans and what size cans are best to use. There seems to be a trend going toward smaller cans to light up entire rooms, one I cannot understand. In my opinion the recessed can is not for looks but for function. They can provide a great source of general illumination in a living space, and they are also great to highlight art or decor.
If you use smaller cans (like 3, 4 or 5-inch) to light up an entire room, you will need to use more cans, and I feel like using more cans creates a look similar to the DON'T picture shown here, it's like you’re looking at stars in a sky. The first problem with the DON'T picture is the layout. To make your cans less noticeable you should lay them out in a grid system, like the DO picture, when your can's are in a symmetrical grid layout you are less likely to pay any attention to them.
When you use 6" cans you most likely will place them every 3', the standard 6" can uses a R30 incandescent bulb or a PAR 30 Halogen bulb. When you use a PAR30 or R30 flood you get 40 degree beam of light. So if you have 9' ceilings with a 40 degree beam of light you will illuminate a space of 6.6-feet in diameter with light. If you use a 4" or 5" can with a PAR20 or R20 flood light you are only getting a 30 degree beam of light. So 9 feet from your light source you are covering a 4.8 feet diameter with light. So you will need to place these cans every 2-2.5' for even lighting.
So if you have a space that is 15' x 15' and you use only 6" cans to light up your space, for an ideal even lighting you would use 9 cans. For that same space if you use the 4" or 5" cans, with a PAR 20 Flood, you would need to use 20 cans to get the same even lighting. That means a lot of holes in your ceiling and a lot more money on the product.
There is an ideal time to use the smaller cans, like to spot light art on a wall or to highlight a workspace, like over your sink in the kitchen. But when you are lighting big living spaces I recommend using the 6" larger cans.