Hi, TSK.
1. The size of your booth is predicated on the size of the work you are planning to do, not the size of the room you are in. Generally speaking, your booth should be selected on the largest kit you plan on painting, then add a couple inches on both sides. The extra inches of clearance help reduce turbulence in the air stream, thereby keeping airflow more laminar.
2. Exhaust ducting should be run from the booth directly outside. Rigging your ducting so you exhaust out the window would be your best bet. Running exhaust ducting into what sounds like the return vent for your HVAC system is a definite no-no. You would just be recirculating vapors back into your entire house. I'm not sure there would be enough vapor concentration to worry about over-exposures to all occupants, since the incoming air would be pretty diluted, but I would imagine the odor would be annoying to all concerned.
3. If you get the proper size fan for the booth you are going to construct, and get the booth set-up correctly, you should be able to spray rattle cans of lacquer without any odors--even in a household environment. Fans are rated according to the amount of air they move in cubic feet per minute (cfm). The amount of cfm you'll need is in direct proportion to the size of the booth you plan on building.
I would recommend you do a search on this forum. Fans and booths have been discussed almost ad nauseum. Do a little research, then come back with your questions. We can help you pick the right fans, help with filter set-ups and exhaust duct runs, and help you build a booth that will fit your space, and keep you and yours safe at the same time.
Waiting to hear from you.
Gip