An End-Fed can be resonant 1/2 wavelength, or multiples. But they can also be resonant 1/4 wavelengths, or multiples. They can also be a random length between 30' to 150'. The difference being the feed impedance. The resonant 1/2 wavelength wires tend to have a feed impedance in the range of 3,500 Ohms and requires a 64:1 Balun/UnUn. Whereas the resonant 1/4 wavelength wires tend to have much lower feed impedance (450 Ohms) and only requires a 9:1 Balun/UnUn. On some bands you might only need a 4:1 Balun/UnUn. Whether you use a Balun or a UnUn, depends on how you are feeding it.
My End-Fed is a variation on a true End-Fed, which brings the feed point into the shack. But bringing the feed point into the shack can cause RF radiation issues. When I operated that way, my RF would get into every speaker and telephone in the shack.
Currently, I have a 92' End-Fed. I use about 20' of RG8x to the feed point at ground level, which has a 9:1 UnUn and is tied to a ground rod. A ground radial might be better, but I haven't gotten around to adding one. The 92' wire travels up to the main support, about 25', and then horizontally for the rest of the run. With a tuner, I can adjust the feed impedance for a 1:1 SWR from 80 to 10 Meters.
An End-Fed antenna has been use for many, many years. Over this span of time, End-Fed users have come up with a group of Magic Numbers. The Magic Numbers are lengths that seem to work very well for them. But for everyone else, YMMV. Below is a small list of some of the Magic Numbers that I have collected.
Magic Numbers for End-Fed with 9:1 UnUn - 40m through 10m
17', 36', 44', 49', 53', 59', 72', 88', 98', 102'
Magic Numbers for End-Fed with 4:1 and 9:1 UnUns - 160m through 10m
58', 71', 84', 107', 119', 148'
Magic Numbers for End-Fed with 9:1 UnUn - 160m through 10m
53', 59', 72', 88.5', 98.5', 124.5', 135', 146'
There are others, like 36', 44', 49', which uses a 9:1 UnUn for 40m through 10m. These lengths are included in the first list, but came from a different source. But I don't remember the source.
So depending on what your band requirements, you can use a variety of lengths that will fit your available space.
Martin - K7MEM