The white shower curtain will shine too much light through, you need to get a black shower curtain to put in between the two white shower curtains that would separate the room.
My best advice would be to hit up HTG (Hightech Garden Supply) for supplies, soil, nutes, etc, as far ducting you can get that at Walmart or Home depot. I recommend getting yourself some panda film or mylar like Crazy said, panda film has a little more reflectivity than mylar. The choice is yours, but you will get better results, also you don't know if the shower curtain will emit a toxic gas when heated, like some Pvc materials can tend to do, which isn't good for your grow at all.
Light leaks are not good, veg you can get away with it a little more, but with bloom you will risk hermie-ing your plants.
As far as size 3x3 like the rest of the club said, you can also do a 2x4.
Make sure for ventilation to have outtake fan high up (Heat rises) and intake fan (Cool sinks) low down. Also you're going to want an oscillating fan inside the grow cab for the plants as well.
You'd be surprised what a 250 can produce, keep your temps at 78F max, 68F low, keep your light 4-6 inches away from the top of the plants, get some decent nutrients, decent soil, and you should be fine.
Hope this helps.