I'm taking over the closet upstairs for a new work area. It's about 42-44" wide. How deep should the workbench be 18' or 24" ? depth will dictate where the bench goes in relation to the door.

----------------------------------------------
|XXXXXXXXXXXXXXXXXXXXXXXXXX|
| 24" XXXXXXXXXXXXXXXXXXXX 18"|
---------------------------- Door ----------

Hi Matt,
Tough question. There are a lot of factors you'll need to consider.
Off the top of my head:
1: Be sure the door can close. Remember to allow for that doorknob. And that your chair will
fit inside as well.
2: Allow room for any items to pass up from below the bench. Things like electrical cords
and the hose from your compressor (if you plan on spraying in there).
3: If you mount shelves above the bench, be sure you have room to work underneath them
without hitting your head.
4: Consider where your overhead lighting hangs. You don't want it behind you or you'll be
working in your shadow.
5: Be sure that you have plenty of elbow room as well.

With these things in mind, I'd recommend that you make it as deep as possible. Better to have too much space than too little!
I hope this was helpful.

Matt
Mongo_mel has hit the major points. Consider spreading out if you need to.

Another thing to consider is how you work. Personally I have a subassembly right in front of me, the other assemblies deeper on my bench and then the immediately used tools to either side. In all my immediate work space is about 24" by 24".

From the picture it looks like the door may open and intrude or impeded the 18" version. If that is the case I'd go 24 just to stay out of the way of the door.

Take a look at the gallery - there are photos of different work areas. That'll give you an idea of whats out there.

I forgot to mention Door opens Out not inward. And there's a bookcase Immediately to the Left of the door on the inside.

Have you considered adequate ventilation? Those glue fumes will make you see pink elephants :-)