I am planning to install a washer and dryer upstairs in a closet. The closet is in a bathroom and has access to services; however, the drain line in only 1-1/2" dia. I know laundry standpipes should be 2" dia, but could the standpipe be 1-1/2" dia? If not... is there a super slim "sink" that I could use?

If 1 1/2" is all that you have I don't see that you have much choice. plenty of older homes, mine included, have 1 1/2" washer stations. I would go for the 1 1/2" and if you have a problem with backups we have a solution for that. Just let us know. Good luck. Tom

... however, the drain line in only 1-1/2" dia.
The washer drain hose is around 1" dia. Itself. So it should just do.
If you get problems you may have to seal the joint of hose and pipe.
This may have some effect of the max. water amount that can be drained per minute.
(in some cases it may even drain faster).

If you run into problems : there are systems on the market to use small drain pipes, even for the toilet!!
(The Sani-Broyeur or Sani-flow range LINK (http://www.youtube.com/watch?v=K5ZTsTzcRa8))

Success !

Just like Tom correctly said, older homes have 1 1/2" drains. But today's washers have high output and may cause overflow. You could always install service sink next to the w/m and drain it to it. Then, you would eliminate the drainage issue.

You can connect the washer drain hose to the
1 1/2 pipe by just sticking it down into the 1 1/2, then if you have a problem with water backing up at that connection you can always make that a solid connection by using a fitting called an anti syphon valve, some may call it a vacuum breaker, this will stop the waste water from the washer from backing up at this joint. This is just something that I know that it will work from experience.

Letrmetellu had the right idea, just the wromg terms.
if you have a problem with water backing up at that connection you can always make that a solid connection by using a fitting called an anti syphon valve, some may call it a vacuum breaker, this will stop the waste water from the washer from backing up at this joint. This is just something that I know that it will work from experience.
What we use to make this a closed system and prevent backups is not a "anti-syphon valve" or a valve of any type. It is certainly not a "vacume breaker" of any type either.
We make the washer discharge a closed system by installing a compression fitting, (see image). Another way would be to simply duct tape the washer hose into the washer stand pipe. Anything's accepotable as long as it seals the hose to the pipe. Good luck and thank you for rating my reply. Tom

Hi all:

Here is the image of the compression fitting Tom was talking about (Tom's image didn't get posted for some reason). In my area we refer to this as a DESANCO fitting.

1.5" pvc is all that is required for a washing machine in Massachusetts... and I have never had an issue as long as the drain pipe or the pipe it was connected to was not clogged. You should be fine as mentioned above.

MARK