ok first things first..
it's completely fine to use a higher wattage resistor, the higher the wattage, means they can take more of a beating. you should almost never base resistor choices on max anything (voltage, amps, etc) you should use the middle of the road to calculate stuff. so if max is 30, and min is 20, use 25. that way if something goes wrong you have a safe zone of operation.
the brightness is really user specific, the best way to do this.. is get the brightest leds you can, then use another method of toning them down to your liking. there is a few ways to do that. you could build the circuit as is, then slightly sand the leds, to get a frosted effect. however some people say that's a bad idea because you have cause deep scratches, that reflect the light back into the led
and severely limit it's lighting ability. another way to diffuse the light, is coat the led
in aquarium sealant. that will diffuse the lite and alot of people prefer doing that, opposed to other methods. next you could take a trip up to hd and get some lighting diffusion planes. it's basically a plastic sheet that diffuses light, you see them alot in commerical buildings that use florescent lighting, alot of people also prefer that method, since it really dissipates the light evenly through the tank and doesn't require you to physcially change the led
my personal opinion though is use to the plastic lighting planes from hd, i think they sold for 4-5bucks and you get a huge pain, 2' x 4'. you cut them to fit your application and they do the best job of diffusing the light.