We recommend a medium pen - in general.  If your pup will be in the pen during a normal workday (8 hours), then having extra space is usually more comfortable.  You can add comfy items such as a bed, a basket of toys, a water bowl, and even a pee pad if needed.
On the other hand, if the pen will just be needed for sleeping at night, a small provides enough space for a bed and a little extra room (depending on the bed size).
Keep in mind that the pens are modular, and you can add a Universal Kit, which can be used as either an Expansion Kit or a Bump Out Kit, to make it bigger at a later date.  However, adding a kit later is more expensive than getting a bigger size to start.
Ultimately several factors must be considered in order to determine what will work best for you including your budget, the space in your house, and your schedule.
*All sizes and heights are available in Silver and Gunmetal Gray.  Blue pens are available in all sizes in the 27" height only.
*All pens come with a walk-through door.