Over at M1ek's I posted a few photos of some Combino trams from different places around the world to show how you can scale the vehicles to fit within any environment. Depending on the capacity needs, there might only use for one streetcar. However, if there are higher passenger volumes, then there are two ways to accommodate this. Either by coupling cars or modular design. Coupling cars is possible, below is a photo of the Skoda T3 in Plzen Czech Republic, the same vehicle as the Portland Streetcar, with couplers.
Photos Courtesy of NYCSubway.org
The next option is the modular tram. Basically, you can build to specifications you want. According to the Siemens website, they c trams from 18 to 72 meters (60 feet to 236 feet) almost 4/5ths of a football field.
Sorry for the bad picture but I stole it from Tom Furmaniak's powerpoint on streetcar tech.
Real Examples below:
Combino - 3 Sections
Flickr Photo by: Johnzebedee
Combino Ultra - 5 Sections
Photo Courtesy of Gen Gibson
Combino Supra: 6 Sections
Flickr photo courtesy of Tschaut
This is the same for the other tram companies including Ansaldo Breda, Skoda, Kinky Sharyo, Bombardier, and Alstom's Citadis Tram.