Wattage Selection!

I've constructed a cabinet using medium density fiberboard for walls, the dimensions being 30"(wide)x22"(deep)x60"(tall)

In terms of cooling, I have a Phresh carbon filter, with a squirrel fan, along with dual 120mm computer fans, both the filters and fans on a temperature controller.

Currently I'm utilizing a 250 watt Metal Halide

I'm convinced I could manage a 400 watt MH, but I'd really like to push for a 600 watt.

Would that simply be too large for the dimensions of the cabinet?

yes, that is a waste of money and it will get too hot.

A general guideline

400 = 3x3
600 4x4
1000 = 5x5

so convince yourself of the truth.. your Cabinet will turn into an easy bake oven with a 600. Even that 400 is going to really turn up the heat in there. COmputer fans are for... well, they are for computers. Your going to need a 4" inline. trust me.

The carbon filter is attached to a 4" inline (squirrel fan). Although, obviously that's less efficient than it could be as it's pulling warm air out, and not pushing cool air in. That being said, I believe you about the 600. I could easily see that, and that's why I asked.

The 400, based on observation, and the fact that the cabinet maintains a constant temperature easily at the moment, still seems doable.

This is my logic;
It takes less than 5 minutes for my cooling array to get the cabinet back down to temperature, in it's current iteration, with a 250 watt MH. It cools maybe twice an hour. I have it set on a 7 degree differential, so it's rather efficient all things considered. Also, bear in mind,your average computer fan is somewhere between 60 and 70mm. Mine are 120mm. Also, the carbon filter contributes to cooling a great deal (there's your inline fan).

If I bumped up the light to 400 watts, I would expect the cooling period to be somewhat extended. Maybe 10 minutes. Though, I'd be curious to see whether it cycles more often.

^so considering all that, do you think I'd nuke my plants with 400 :/?

I grow in a closet with one 400 it is 8 feet tall x 4 long x 2 feet wide and I basically utilize a 2x3 grow area. I have a vented hood and inline fan at 160 cfm pulling air through the filter, through the hood and out to my exhaust in the attic. It runs 24x7 at full speed and I maintain around 80 in that closet. Ambient temps can make that closet go up or down 5 degrees. I have a passive intake from the negative pressure pulling cooler air from room. I have two 6" clip on fans that blow in the closet as well. If I turn my inline fan down or off my temps get up 95 pretty quick. If you do make the jump and try a 400/600 make sure you get a dimmable ballast just in case! I am in Cali and have outside temps ranging from 65-80 degrees during the day most of the time. In the heart of summer when it is 85-95 either my AC is pumping or I just don't grow in the closet.

Ahh didn't know you had an inline fan running on there.

Pulling the hot air out is actually
More efficient than blowing cool air in...it at least in my experience. If you can truly get temps down that fast then it sounds like it will work. You have to remember though that your basically doubling the heat in there and it's not a linear correlation. Meaning that a doubling of the heat source is not neccessarily going to be handled by a doubling of the time the fans are on.