The brightness, or lumens is determined by the bulb you select not the size of the fixture. For example: a 65w bulb with 500 lumens for a 5" fixture shines just as brightly as a 65w bulb with 500 lumens shines for a 4" fixture. The difference, is the shape of the "cone of light" that it casts. Clearly a 4" fixture casts a cone of light smaller than a 5". The smaller the light, the more fixtures you will need. The following link describes it pretty well.

http://recessedlighting.com/recessed-lighting-spacing/

It basically comes down to placing them at the correct intervals so the bottom of your cone of light intersect on the surface you wish to illuminate. I also recommend dimmers on recessed fixtures. It gives you more flexibility since you're not likely to want full blast lighting all the time. It will save on your electric bill too.

...speaking of saving on your electric bill, here are a couple tips I posted earlier this month. It may help if you are considering LED lighting.

LED Lighting tips- When selecting any LED lighting there are two things to note: 1. The color temperature (expressed in degrees Kelvin)- The higher the number, the cooler the light will appear. 4000k to 6000k is more blue and white. The closer to 2700k, the more it will resemble the warm tones of an incandescent bulb. While the color temperature Is important, it's not quite as important as... 2. CRI or Color Rendering Index- The closer to 100, the more accurate the color of the light produced will be. A CRI of 80, for example, will make things in the room wash out almost like a fluorescent light, which is not what you want if you've spent any time selecting colors of paint or fabrics. While a CRI of 95 will be very similar to the light of an incandescent bulb, warm and inviting.