I'm assuming that if I bought a 15 meter slackline, I could actually tie it between two trees at any length less than 15 meters apart and use it. Please tell me if that assumption is wrong. So what's the minimum gap (distance between two trees) I need if I just want to learn as a beginner to step on the slackline without falling off and walk a few steps?
As close together as you want! Most climbing gyms will set up 5m (ish) lines which is a pretty good distance for beginners. In general you probably don't want to go much less than 3m. Keep it nice and taught while you are learning; the less slack in the line the easier it will be.
Most of all just get it set up and have fun. It doesn't need to be set up perfectly to enjoy slacklining. The learning curve for slacklining is really quick. Just keep your head up, arms up and out, bend the knees, and breathe. You'll be walking on it in no time!
Slack lining is actually a lot easier in the beginning the shorter your line is, as there will be less movement of the line (side-to-side wobble or up-and-down bouncing). There isn't really a minimum length for a slack line - personally, when I started out I sometimes set up a line no longer than some 3m...
So my suggestion is to find some nice trees not too far apart (maybe start with 4-5m) and once you're comfortable on that distance start increasing it.
As you seem to be new to slack lining and this question might attract other beginners, I'll add the usual tree protection advice:
Slack lining can damage trees and their bark, especially in public parks, and other areas where people frequently set up lines. There are some things to keep in mind that help minimise any damage:
- Please pick trees that are strong enough, as the sidewards force of a taut and loaded slack line can be considerable. A common suggestion ist that the trees should have at least 30cm of diameter of the trunk (ca. 1m of circumference) at the height where you want to setup the line.
- Between your slack line / loops and the tree bark there should always be some cushioning material, to help protect the bark from the applied forces. What I have seen in use / or used myself: a strap of an old carpet, a folded up bathing towel, an old rubber doormat. Of course there also exist dedicated solutions that you can buy (check google).
- Some also suggest using a setup with no contracting loops around the trees as contracting loops can damage the tree bark. (This means you'd need two fixed-length loops to wrap around the trees and would set up your line between those loops.)
More info here.