Some of the tunings in this site are impossible with normal strings and scale lengths. For example, some seven and eight string guitars are shown as having a top string tuned to a', 440Hz.
There are two problems with high tunings such as this:
There's an obvious solution: Use a shorter scale. And many of the extreme tunings you see for guitars for example may well be used on short-scale guitars such as the (rare) 22.5-inch version of the Fender Mustang, or a custom built special, perhaps with fan frets so the treble strings are shorter than the bass.
But there are other solutions too.
Any twelve string player will tell you it's hard enough to find steel strings that tune to g' reliably, let alone two semitones above it.
The problem is, if you increase the cross-sectional area of the string in order to make it stronger, you also increase its mass per unit length so you need more tension for the same pitch, and the two cancel out, almost exactly in fact. Similarly, if you decrease the string gauge it reduces the tension required, but also the string strength.
These tunings are possible, but only by using special strings. See for example
for one maker. Octaveplus strings currently go down to four thou, .004 (inches understood when talking here of string gauges).
To put that in perspective, the coursest human hair is about ..007", and the finest about .0007". Few conventional string sets go thinner than .009, many string makers stop at .008, and no conventional steel guitar string is finer than .007. But Octaveplus make .006 and .005 in their standard series, and .004 by special order.
On a standard 25" scale guitar, Octaveplus claim that their strings will tune up to c' ' provided you allow about an hour to stretch the string gradually.
With nylon strings the problem is not so much the strength of the string but that the guitar is built for a lot less tension, so although the string won't break the guitar might. Again, special strings required.
Lenny Breau, who made the high A tuning famous for the seven string guitar, made his own extra-light top A strings from fishing line.