You should think about 3 things when you buy a slackline:

1. Length
2. Width
3. Brand / System

Length
Choosing the length is easy. Most lines are either 15 meter or 25 meter. This is the maximum length, or span, that your slackline will cover. So a 15 meter set can be setup between 2 anchor points (trees, poles, posts etc) that are up to 15 meter apart.
Being the maximum span, you can also setup a 15 meter slackline or anything shorter.

Width
The width of the line/webbing is 25mm, 35mm or 50mm. Most beginner lines are 50mm. In fact most lines are 50mm. The width does make a slight difference to the difficulty, with the wider lines being easier to walk on. The wider lines are also more comfortable under bare feet.

Brand / System
There are a number a great brands now in slacklines. The biggest two currently are Elephant and Gibbon. Both companies are based in Germany. The Classic White Slacklineshop NZ slacklines are 100% made in New Zealand.