I have a small one that's 5000 BTU , but too small for our room. I'll probably use that in my son's room.

Needs to be 120 volt, and able to cool 400ish sq ft with high ceilings.

Lowes has a Frigidaire  10,000 BTU for $299. Says up to 450 sq ft.

Can't find the wattage listed under specs though . My little one is listed at 600 watts. So maybe double that?

Yep - Should be pretty close to double. Note that the starting surge will be a lot more than the rated running load - which may give your generator trouble.

For that reason, it might make more sense to use a second 5000 btu unit instead - you can wait for the generator to start the first one before you turn on the second.

For short-term use, even a grossly undersized window unit makes a room a lot more comfortable - Reducing the humidity is one reason why. Add a box or table fan (or temporarily wire your ceiling fan to the generator) to keep air moving.

Well I FO'd

Bought a 12000Btu Frigidaire.  Hope I don't regret it , not knowing the wattage.
The 10000btu wasn't available at the lowes near me.

I got too excited and didn't want them sold out like last time for Ike.

Probably a good idea going bigger, can always turn it down if it's to cold.  

Probably between 11-1300 watts depending on efficiency.  Surge will be more of course 

I have a DeLonghi 12,500 btu standing room A/C that has a flexible hose that goes to a panel you vent out the window.  It does a pretty good job at cooling just my bedroom and bathroom when my upstairs heat pump went out in May and it was 85 outside, and it was keeping those 2 small rooms at about 74 degrees.  10 amps, 115 volts.  It's a great emergency HVAC unit for one room like this.  I used a similar unit downstairs a few years ago when my downstairs heat pump quit, and it did OK, since the heat flows upstairs to where I still had cooling at that time.  My downstairs has no doors to seal off rooms, and is approximately 900 sq ft.